Add (basic) support for transfer functions to Images (issue 6931, bug 1149713)

This is *similar* to the existing transfer function support for SMasks, but extended to simple image data.
Please note that the extra amount of data now being sent to the worker-thread, for affected /ExtGState entries, is limited to *at most* 4 `Uint8Array`s each with a length of 256 elements.

Refer to https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G9.1658137 for additional details.

test/pdfs/.gitignore

@@ -45,6 +45,7 @@
 !issue7492.pdf
 !issue7544.pdf
 !issue7507.pdf
+!issue6931_reduced.pdf
 !issue7580.pdf
 !issue7598.pdf
 !issue7665.pdf

test/test_manifest.json

@@ -4468,6 +4468,12 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue6931",
+       "file": "pdfs/issue6931_reduced.pdf",
+       "md5": "e61388913821a5e044bf85a5846d6d9a",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "annotation-button-widget-annotations",
        "file": "pdfs/annotation-button-widget.pdf",
        "md5": "5cf23adfff84256d9cfe261bea96dade",