[Impeller] Usage of framebuffer fetch has unnecessary render to texture.

[jonahwilliams]

To render advanced blends, even with framebuffer fetch enabled we unconditionally do a render to texture on the contents and then use a framebuffer fetch advanced blend shader with the resulting texture.

This is better than a backdrop filter but still slower than if we drew the original contents (solid color, text) with blending applied in the shader.

Unfortunately today doing so would add (# of color sources) * (number of advanced blends) additional shaders. We should investigate if static shader variants via specailzation constants (#119357) could allow us to include more shaders without both 1) increasing bundle size and 2) re-introducing shader jank.