Angry Birds 2 is the first app we've targeted that maps buffers
across frame boundaries.  This exposed some gaps and assumptions
we had in the code, and required additional support for MEC.

To support this, we track each buffer's starting map/unmap state
and how it changes throughout the trace.  Then during Reset, we emit
calls to return them to the correct state:

void ResetContext3Replay()
{
    ...
    glBindBuffer(GL_ARRAY_BUFFER, gBufferMap[546]);
    glUnmapBuffer(GL_ARRAY_BUFFER);
    glBindBuffer(GL_ARRAY_BUFFER, gBufferMap[550]);
    gMappedBufferData[gBufferMap[550]] =
        glMapBufferRange(GL_ARRAY_BUFFER, 0, 8192, GL_MAP_WRITE_BIT);
    ...
}

Test: MEC of Angry Birds 2
Bug: angleproject:4599
Bug: b/157672184
Change-Id: I5c73ca4d4eba7f1ecea01467ae887bae7f2d27fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2231803
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>