dc1394: cache ROI boundaries and simplify set_roi() function.
dc1394: cache ROI boundaries and simplify set_roi() function.

By caching the ROI boundaries, the get_frame_roi() function no longer
needs to call libdc1394 and thus avoids any potential syscalls. This
has the side-effect of making signals that trigger EINTR to be much
less likely.

Additionally, the code in set_frame_roi() was simplified.

motmot/libcamiface,motmot/libcamiface,motmot/libcamiface,motmot/libcamiface