formats: Enable more formats for readback

Gert Wollny requested to merge gerddie/virglrenderer:enable-more-readback into master

On OpenGL all textures can be read back using glGetTexture but for GLES we have to be able to bind the texture as render target, and glReadPixels must support the format, so the logic for GLES stays the same.

It may be possible to rely on the data that is stored in the IOV (the tweak proposed in mesa/mesa!13572 (merged) proves this for CS:GO), but there is a number of reasons why we can't rely on this data always being valid, that's why I didn't touch the GLES logic with this version patch.

Edited by Gert Wollny

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