Move svg:clipPath generation from clip to endPath

In the PDF from issue 8527, the clip operator (W) shows up before a path
is defined. The current SVG backend however expects a path to exist
before generating a `<svg:clipPath>` element.
In the example, the path was defined after the clip, followed by a
endPath operator (n).
So this commit fixes the bug by moving the path generation logic from
clip to endPath.

Our canvas backend appears to use similar logic:
`CanvasGraphics_endPath` calls `consumePath`, which in turn draws the
clip and resets the `pendingClip` state. The canvas backend calls
`consumePath` from multiple other places, so we probably need to check
whether doing so is also necessary for the SVG backend.

I scanned our corpus of PDF files in test/pdfs, and found that in every
instance (except for one), the "W" PDF operator (clip) is immediately
followed by "n" (endPath). The new test from this commit (clippath.pdf)
starts with "W", followed by a path definition and then "n".

    # Commands used to find some of the clipping commands:
    grep -ra '^W$' -C7 | less -S
    grep -ra '^W ' -C7 | less -S
    grep -ra ' W$' -C7 | less -S

test/pdfs/issue6413.pdf is the only file where "W" (a tline 55) is not
followed by "n". In fact, the "W" is the last operation of a series of
XObject painting operations, and removing it does not have any effect
on the rendered PDF (confirmed by looking at the output of PDF.js's
canvas backend, and ImageMagick's convert command).

src/display/svg.js

@@ -361,6 +361,7 @@ SVGGraphics = (function SVGGraphicsClosure() {
     this.extraStack = [];
     this.commonObjs = commonObjs;
     this.objs = objs;
+    this.pendingClip = null;
     this.pendingEOFill = false;
 
     this.embedFonts = false;
@@ -389,6 +390,7 @@ SVGGraphics = (function SVGGraphicsClosure() {
       this.transformMatrix = this.transformStack.pop();
       this.current = this.extraStack.pop();
 
+      this.pendingClip = null;
       this.tgrp = null;
     },
 
@@ -894,9 +896,10 @@ SVGGraphics = (function SVGGraphicsClosure() {
       current.setCurrentPoint(x, y);
     },
 
-    endPath: function SVGGraphics_endPath() {},
-
-    clip: function SVGGraphics_clip(type) {
+    endPath: function SVGGraphics_endPath() {
+      if (!this.pendingClip) {
+        return;
+      }
       var current = this.current;
       // Add current path to clipping path
       var clipId = 'clippath' + clipCount;
@@ -905,17 +908,18 @@ SVGGraphics = (function SVGGraphicsClosure() {
       clipPath.setAttributeNS(null, 'id', clipId);
       clipPath.setAttributeNS(null, 'transform', pm(this.transformMatrix));
       var clipElement = current.element.cloneNode();
-      if (type === 'evenodd') {
+      if (this.pendingClip === 'evenodd') {
         clipElement.setAttributeNS(null, 'clip-rule', 'evenodd');
       } else {
         clipElement.setAttributeNS(null, 'clip-rule', 'nonzero');
       }
+      this.pendingClip = null;
       clipPath.appendChild(clipElement);
       this.defs.appendChild(clipPath);
 
       if (current.activeClipUrl) {
         // The previous clipping group content can go out of order -- resetting
-        // cached clipGroup's.
+        // cached clipGroups.
         current.clipGroup = null;
         this.extraStack.forEach(function (prev) {
           prev.clipGroup = null;
@@ -926,6 +930,10 @@ SVGGraphics = (function SVGGraphicsClosure() {
       this.tgrp = null;
     },
 
+    clip: function SVGGraphics_clip(type) {
+      this.pendingClip = type;
+    },
+
     closePath: function SVGGraphics_closePath() {
       var current = this.current;
       var d = current.path.getAttributeNS(null, 'd');

test/pdfs/.gitignore

@@ -90,6 +90,7 @@
 !issue3879r.pdf
 !issue5686.pdf
 !issue3928.pdf
+!clippath.pdf
 !close-path-bug.pdf
 !issue6019.pdf
 !issue6621.pdf

test/pdfs/clippath.pdf

@@ -0,0 +1,37 @@
+%PDF-1.1
+1 0 obj
+<</Type/Catalog/Pages 2 0 R>>
+endobj
+2 0 obj
+<</Type/Pages/Count 1/Kids[3 0 R]/MediaBox [0 0 200 100]>>
+endobj
+3 0 obj
+<</Type/Page/Parent 2 0 R/Contents 4 0 R>>
+endobj
+4 0 obj
+<</Length 65>>
+stream
+W
+40 20 m
+160 20 l
+160 80 l
+40 80 l
+h
+n
+0 0 0 sc
+0 0 200 100 re
+f
+endstream
+endobj
+xref
+0 5
+0000000000 65535 f 
+0000000009 00000 n 
+0000000054 00000 n 
+0000000128 00000 n 
+0000000186 00000 n 
+trailer
+<</Root 1 0 R/Size 5>>
+startxref
+299
+%%EOF

test/test_manifest.json

@@ -2776,6 +2776,14 @@
       "type": "eq",
       "about": "CFF font that is drawn with clipping."
     },
+    {  "id": "clippath",
+      "file": "pdfs/clippath.pdf",
+      "md5": "7ab95c0f106dccd90d6569f241fe8771",
+      "rounds": 1,
+      "link": false,
+      "type": "eq",
+      "about": "Clipping before a path exists, followed by adding a path and then drawing a rectangle."
+    },
     {  "id": "annotation-tx",
       "file": "pdfs/annotation-tx.pdf",
       "md5": "56321ea830be9c4f8437ca17ac535b2d",