Compressed texture atlas support on the QRhi-based rendering path

Feature missing atm, currently compressed textures are never atlased when using the rhi.

(compressed textures themselves are supported but they won't be automatically atlased, no matter what)

While not essential, strictly speaking this would be a regression from the direct OpenGL path of 5.x.

Also, removing the qsgopenglatlastexture cpp and header files are not straightforward until a solution is reached here because QSGCompressedAtlasTexture depends directly on QSGOpenGLAtlasTexture. Hence adding this as a dependency for QTBUG-79268.