[Linux] Fix incorrect texture format with OpenGL ES

[huanghongxun]

This PR fixes #76881.

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[huanghongxun]

This pull request fixes embedder complains about render target creation failure.
Please note that this patch only fixes incorrect texture format. After applying this patch, the Flutter window unexpectedly whitens, which I have no idea for fixing.
@robert-ancell @wmww

[huanghongxun]

@chinmaygarde I want to know why embedder uses a platform-dependent texture color type instead of a certain value (e.g. RGBA8888). Linux shell must decide texture format according to platform(CPU endian according to skia GrGLCaps.cpp) instead of settings a certain value(e.g. GL_RGBA8), which results in duplicated code with Skia or importing Skia headers.

engine/shell/platform/embedder/embedder.cc

Line 462 in d5058fb

kN32_SkColorType, // color type

Changing above constant to kRGBA_8888_SkColorType will result in a simpler solution in Linux shell.

[robert-ancell]

LGTM, thanks @huanghongxun!