[Impeller] Fix framebuffer blend shader validation errors on Linux GLES

[whiskeyPeak]

The impeller backend on linux was throwing errors due to not having had the neccessery shaders linked up. This PR fixes that.

Error: [ERROR:flutter/impeller/base/validation.cc(49)] Break on 'ImpellerValidationBreak' to inspect point of failure: Could not resolve pipeline entrypoint(s) 'framebuffer_blend_vertex_main' and 'framebuffer_blend_fragment_main' for pipeline named 'FramebufferBlend'.

[flutter-dashboard]

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[jonahwilliams]

@whiskeyPeak WDUT about trying to write a test for this? I think in theory if you had an emedder unit test that enabled impeller, you could assert that it initialized correctly with this change?

[whiskeyPeak]

@jonahwilliams I'm having trouble figuring out how to go about this. I looked at tests in embedder_gl_unittests.cc and there doesn't seem to be a way to check whether a rendering context has been initialised correctly.

I also can't enable impeller for single tests because the logic that decides what rendering context is used (impeller or skia) is in the FlutterEngineRun() function, so I would need to pass down my own Settings object which would break the embedder api. Though most tests fail when I enable impeller globally, even with this PR

[jonahwilliams]

Rather than adjusting things via settings and constructing the entire embedder, could you create a new unit test that just tries to create a EmbedderSurfaceGLImpeller?

[chinmaygarde]

Any update on a test for this?

[whiskeyPeak]

Looks like this fix was incorporated in #50416. I'll close this here.