Rename PatternType to ShadingType to avoid confusion
The current name is somewhat confusing, since the specification calls it `ShadingType`, see http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G7.4044105 and http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G7.3882826.
The real problem, however, is that there is actually another property called `PatternType`, which makes the current code very confusing, see http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G7.1850929.
Since `ShadingType` is only relevant for shading patterns (i.e. `PatternType === 2`), and *not* for tiling patterns (i.e `PatternType === 1`), this patch should help reduce confusion when reading the code.
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