Consolidate GPU buffer implementations

Consolidates all the different buffer implementations into a single
GrBuffer class. This will allow us to add new buffer types, use DSA in
OpenGL, track buffer bindings by unique ID, cache buffers without
respect to the type of data they have been used for previously, etc.
This change is strictly a refactor; it introduces no change in
functionality.

BUG=skia:
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Review URL: https://codereview.chromium.org/1825393002
69 files changed