vrend: Use pixel pack buffers as buffers and unbind after use
On one hand it is simpler on GLES, because it allows direct readback, and on the other hand it fixes failures when EXT_texture_buffer is not available.
Also unbind the buffers after use because otherwise glReadPixel would interpret the pixel pointer argument as offset into the pixel buffer and would fail.
Closes: #45 (closed)
Signed-off-by: Gert Wollny gert.wollny@collabora.com