multigpu: Consider other formats for transfer than the original

[Drakulix]

This is an attempt to make the multigpu renderer a bit more flexible, if the sets of formats of two gpus just don't align.

Firstly it makes the dma_shadow_copy try more formats, if the original one isn't available. First it tries to find one with the same bpp (falling back to 8, if it can't be determined, which allows us to finally display something for YUV-formats, even if not accurate) and then it accepts losing precision down to 8-bits per color.

Secondly if the mem_copy-code fails to read from a texture, we currently have no fall back. ExportMem::can_read_texture is meant to prevent that, but on OpenGL ES we can only try to truly see if reading works. Sadly the nvidia driver doesn't allow us to read with GL_RGBA and GL_UNSIGNED_INT_2_10_10_10_REV (it falls with INVALID_OPERATION) even though the spec says it should(?): https://docs.gl/es3/glReadPixels.

This is in particular a problem with games not going through PRIME (e.g. via wine-wayland) which like to allocate their framebuffer in XBGR2101010. So as a fall back we render these into a Abgr8888 shadow buffer and copy from there (which works fine in my testing, even though we obviously lose some precision).

Ideally we should have a vulkan renderer, where none of this is an issue and we can use dmabuf-sharing even with nvidia with Linear-modifiers, but in the meantime this is better than a black window.