[pdf] Clean up make_function_shader

Handle enums with switch cases in order. Add a ShadingType enum to
clarify magic constants. Move two point conical code to only be run
when needed. Add basic support for "decal" tiling mode.

Change-Id: I17bd2e7599139b4a4aee2bdf6c2d9a2b4f294ab4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/851990
Auto-Submit: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed