When mapping buffer memory, a flush is performed if the buffer has
pending operations followed by a finishToSerial to make sure the buffer
is no longer in use by the GPU.

This also implements GLES 3.0 buffer mapping flags:

GL_MAP_INVALIDATE_RANGE_BIT: No-op. Vulkan's vkMapMemory doesn't have
such a feature.

GL_MAP_INVALIDATE_BUFFER_BIT: Same

GL_MAP_FLUSH_EXPLICIT_BIT: Vulkan automatically flushes host memory
writes on vkQueueSubmit, so this is no-op as well.

GL_MAP_UNSYNCHRONIZED_BIT: The flush+finishToSerial call is skipped in
this case.

Bug: angleproject:3213
Change-Id: I6bdb460dffbb57170649f4c9678afbfae331926c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1661252
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>