Fix usage enum with GL_NV_pixel_buffer_object.

This extensions adds PBOs but not new enum values for <usage>. Only the
ES2 enum values of STREAM_DRAW, STATIC_DRAW, and DYNAMIC_DRAW are valid.

Also change TransferBufferType to enum class.

Bug: skia:1040643
Change-Id: Id36808267a86548a25ac6c92f9b753c11764742e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265577
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed