Carry exposure over when switching from RGB to jpeg modes

Right now the jpeg image exposure is wrong, not matching what was seen on the LCD. Figure out what registers to twiddle (save & restore?) so that the exposure is preserved and the jpegs are nicely exposed.

Start by tracing the Python driver’s RGB-to-JPEG mode switch and identify how exposure is represented in the OV2640 registers. Compare the exposure used for the LCD view with the JPEG-mode configuration; done means JPEG output preserves the observed exposure and is consistently well exposed.

Written by the indexing model from the issue text.