50D: fixed starting location of raw-photo buffer and fixed skip offsets

Created originally on Bitbucket by ayshih (Albert Shih)

Was already merged in Bitbucket before import, marked as merged by the import user

The address of the raw-photo buffer for the 50D was off by two 4832-pixel rows plus an additional 16 pixels. The 16-pixel shift is readily apparent, while the two rows were determined by checking against output from dcraw -E. The shift was throwing off black-level estimation (specifically, the standard-deviation-based measurement of noise) because the regions normally used would not be well within the black bars.

I also changed the skip offsets so that they point to the start of real image data; these match the offsets used by dcraw, and I separately confirmed them. If the skip offsets are instead intended to point at the beginning of the nominal image area that is the normal output of processing the CR2 (i.e., slightly cropped), then the older values are nearly correct (there are off-by-one errors that still need to be fixed). Let me know which way it should be.