Fix GBM color format

[HidenoriMatsubayashi]

I fixed the color format value because the value specified in gbm_surface_create was incorrect. I don't know why but it worked on RPI4 etc.

See: https://www.khronos.org/registry/EGL/extensions/MESA/EGL_MESA_platform_gbm.txt

Related issues

• Test on arm devices #4

[HidenoriMatsubayashi]

@taka-okayama Will you review this PR?

[taka-okayama]

@HidenoriMatsubayashi

I have a question about pixel format.
Why do you choose GBM_FORMAT_ARGB8888 format? This format has ALPHA component, so this effects alpha blending in every rendering frames. I think GBM_FORMAT_XRGB8888 is a better choice than ARGB such as EGL_MESA_platform_gbm examples.

[HidenoriMatsubayashi]

Because Flutter Embedder uses Alpha. See: https://github.com/sony/flutter-embedded-linux/blob/master/src/flutter/shell/platform/linux_embedded/surface/context_egl.cc#L23. Also, Flutter UI planes can overlap with other planes such as Video.

[taka-okayama]

Because Flutter Embedder uses Alpha. See: https://github.com/sony/flutter-embedded-linux/blob/master/src/flutter/shell/platform/linux_embedded/surface/context_egl.cc#L23. Also, Flutter UI planes can overlap with other planes such as Video.

I got. thank you.