Carry exposure over when switching from RGB to jpeg modes

[jepler]

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.

[jepler]

I found some info about switching to a manual exposure mode: https://www.arducam.com/manual-exposure-ov2640/