The old condition happened to work, because SFLAG_INTEXTURE was not
set(we're loading it), and neither was SFLAG_INDRAWABLE(otherwise
INTEXTURE would be set too). If the fbo INDRAWABLE == INTEXTURE is
replaced by INDRAWABLE == INSRGBTEX this doesn't hold true any longer
because SFLAG_INDRAWABLE is set, but not used because the drawable
readback is never used for fbos.

FBO stretchrect marks SFLAG_INDRAWABLE up to date, which currently
also marks SFLAG_INTEXTURE up to date. However, this will change when
we enable rendering to the srgb copy, in which case the drawable could
be equal to the srgb copy, not the rgb copy.

The D3D context is potentially destroyed, so we need to clear the thread's
current D3D context. Found by Rico Schüller.

This is just EXT_provoking_vertex promoted to ARB.