[Impeller] Determine whether framebuffer fetch via subpasses will be supported in the GLES backend #144181
Description
The framebuffer fetch implementation was extended in flutter/engine@f4fbabf and now relies on capabilities that are not supported in the Impeller OpenGL ES back end.
For example, running RendererTest.CanSepiaToneWithSubpasses fails on GLES because the resulting shader requires a sampler2DMS that is not supported on a #version 100 shader.
If supporting this on GLES is not feasible, then we may need to change the capabilities to reflect that GLES does not match Vulkan/Metal's level of support for framebuffer fetch.