`Texture::grab_screen` format

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When testing with the Macroquad examples, I find that the screen_texture example fails on the EGL backend, due to Texture::grab_screen uses the texture format (which is the default RGBA in this case) instead of the screen frame buffer format (RGB). The example works after setting conf.platform.framebuffer_alpha to true.

So Miniquad should expose the frame buffer format in some way.