gl2 encoder: async unmapbuffer

bug: 140112486

- No need to talk to the host if we glMapBufferRange with invalidation.
- No need to talk to the host if we glUnmapBuffer without wanting to
write.

Use the extension string to decide which to use, as it may or
may not be available on the host.

Change-Id: Ib50d24064d4c4d5517dc02b4d65f0f314f5d4ccf
11 files changed