Re-allocate the rawImage buffer if the number of channels has changed.

Summary:
If the number of channels changes during a session (rare, but can happen now that the
simulator can be changed from mono to rgb and back) then the rawImage buffer needs to be
re-allocated.

Test Plan:
Using the new Indi ccd simulator capability to switch between rgb and mono,
if you do this in one session without this fix, you'll see the fitsviewer
not display the image properly after the change (though it would it you saved configuration
and restarted kstars). With this fix, that bug should go away--e.g. change the
ccd simulator to rgb, capture an image, note the display is reasonable, switch again, should
be reasonable again.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26778