PixelCopy: align with legacy GLConsumer::computeTransformMatrix

Chroma channels of YUV420 images are subsampled we may need to shrink
the crop region by a whole texel on each side. Since skia still adds
its own 0.5 inset, we apply an additional 0.5 inset. See
GLConsumer::computeTransformMatrix for details.

Bug: 204725913
Test: android.media.cts.DecodeAccuracyTest
Test: android.view.cts.PixelCopyTest
Change-Id: I38bd3d1e5f4c62fff3dd0374c3f2500f1a2d90b9