Support deferred GPU images in ImageShaders

[dnfield]

• De-skiafy image shaders and layers
• Turn the runtime effect color source into its own object/op
• Add a TODO for applying this to text. Image shaders using GPU images on text foreground or background will not work yet.

Fixes flutter/flutter#109286 Fixes flutter/flutter#105085

Creates a new TODO related to https://github.com/flutter/flutter/issues/95434, specifically around the need for SkParagraph to be decoupled from SkPaint.

@jonahwilliams @c-h-i-a-m-a-k-a-2 FYI - patching this in for local testing should enable image shaders for pattern support the way we want to use them in vector_graphics.

Working on this is making me realize that ImageShader is lacking a dispose method and it should definitely have one. Tracked as part of https://github.com/flutter/flutter/issues/82832.