Selectively disable filtering if estimated safe

If the output matches the source rect then use
GL_NEAREST instead of GL_FILTER. This is a more
pixel-exact capture.

Bug: 38242146
Test: CtsUiRendering & CtsViewTestCases:.PixelCopyTest passes on fugu
Change-Id: I9f57a4124374568f83d45fdc8f83cc767ded888a