Improve performance of reused patterns.

Bug 1721218 has a shading pattern that was used thousands of times.
To improve performance of this PDF:
 - add a cache for patterns in the evaluator and only send the IR form once
   to the main thread (this also makes caching in canvas easier)
 - cache the created canvas radial/axial patterns
 - for shading fill radial/axial use the pattern directly instead of creating temporary
   canvas

test/pdfs/.gitignore

@@ -62,6 +62,7 @@
 !issue7872.pdf
 !issue7901.pdf
 !issue8061.pdf
+!bug1721218_reduced.pdf
 !issue8088.pdf
 !issue8125.pdf
 !issue8229.pdf

test/test_manifest.json

@@ -5623,6 +5623,12 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "bug1721218_reduced",
+       "file": "pdfs/bug1721218_reduced.pdf",
+       "md5": "20ee3e9500d8f5f5e532ec2edfaaaaaa",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "pr8491",
        "file": "pdfs/pr8491.pdf",
        "md5": "36ea2e28cd77e9e70731f574ab27cbe0",