SurfaceTexture: shrink all sides when cropping

This change makes SurfaceTexture include an offset for all sides of the crop
region when cropping.  This keeps the image centered, to minimize the visual
changes when switching between the texture transform matrix-based cropping and
something that does proper cropping (e.g. HWComposer).

Change-Id: I541d3046fd92e49221b488444df36d490924d1c5