vrend: fix bgrx->rgb fallback data upload for GLES hosts

Fix the swizzle, reconfigure the unpack alignment/stride, use the
correct GLenum `GL_RGB` for data upload API calls, add TODO to implement
also for the missed readback path.

Fixes: 4717292c ("vrend: deal with BGRX upload on GLES")
Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <virgl/virglrenderer!1352>