Was performing context access per object for scene, region etc.
While not very slow they don't change during drawing so cache in a struct.
This also makes it possible to draw in views besides the current context.
Same as MEM_SAFE_FREE macro,
checks for NULL, runs free then sets NULL.
Blocks of code that do this many times are noisy and likely
errors here wouldn't be noticed immediately.
Also NULL's static vars which were being left set.
This removes MAX_STORAGE, MAX_BUFFERS, MAX_TEXTURES, MAX_PASSES limits.
Actual memory saving isn't so important, it just means we don't need to
manually bump these based on changes to engines.
