mesa: release references to image textures when a context is destroyed

mesa: release references to image textures when a context is destroyed

When a texture is still bound as an image and the context it was bound in
is destroyed but not the texture, then the texture will still hold the
resource and will not be freed when it is finally destroyed. Hence, release
these references when the context is destroyed.

This leak was triggered by virglrenderer:
https://gitlab.freedesktop.org/virgl/virglrenderer/issues/86

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>