fix SF buffer cropping

When a buffer had a crop (meaning its content is scaled to the window size)
and a window crop was defined, the resulting crop couldn't be expressed
properly because h/w composer's API was limited to integers, since
this is fixed in h/w composer 1.3, we take adventage of this to
make sure we get the correct crop.

this bug could result in the buffer being scaled by an incorrect ratio and
be slightly offset; moreover, it would produce different results from the
GL code path, which is always correct.

Change-Id: I8e20e00b6e26177d14f4ab4d2cd581e26c818892