Support copying from index8 into another index8.

Both bitmap's must share the same color table as we essentially
memcopy the pixel data between the buffers.

Bug: 64112829
Test: CtsGraphicsTestCases
Merged-In: I317e3a814dbd102925fe412d2e19111b0af3af38
Change-Id: I5d99627fe7dd0639bb47a2eb8790510beebe6c6f