opengl: fix and refine memory barriers

The current memory barriers were either used illegally
(tex_gl->target??), overspecified (e.g. the forced barrier after
glDispatchCompute), or just plain missing (e.g. barriers for storage
images that might also be used in other ways).

Fix this mess by making the barrier usage consistent:
1. Only submit memory barriers on descriptors when unbinding them from
   shaders that might write to the resource
2. Skip unnecessary memory barriers for readonly resources
3. Submit *all* the barriers required for the object's usage flags