mesa/meta: Support decompressing floating-point formats

mesa/meta: Support decompressing floating-point formats

Previously the Meta implementation of glGetTexImage would fall back to
_mesa_get_teximage if the texturing is not using an unsigned normalised
format. However in order to support the half-float formats of BPTC textures we
can make it render to a floating-point renderbuffer instead. This patch makes
decompression_state have two FBOs, one for the GL_RGBA format and one for
GL_RGBA32F. If a floating-point texture is encountered it will try setting up
a floating-point FBO. It will now also check the status of the FBO and fall
back to _mesa_get_teximage if the FBO is not complete.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>