memory optimizations
we currently keep a number of bo's per-ctx rather than per-screen:
blit_mem-
vsc_data/vsc_data2
However these are (at least currently) only used within the context of a batch, ie. values don't matter across batches. We could save some memory in cases where there are many contexts by moving these to the screen.
However, if we do ever decide to enable intra-batch/submit context switches, we probably want to have these buffers per-priority level (so different contexts hanging off same screen, but with different priority levels, don't stomp on each other). Probably the thing to do is have some static-inline fd6_context_foo_mem() accessors which grab the bo from the screen, but later could be extended to grab the appropriate per-priority-level bo from the screen.
How useful this would be depends on how many contexts are created off a single screen, but I guess w/ the browser that could be a lot.