cacorrect: fix a segfault for very small buffers.
color picker: backtransform picker coordinates; fixes #9482

coordinates of local color picker need to be transformed back from the preview
image to the module in focus, else we pick completely bogus spots if distorting
modules are in action.
color picker: better account for invalid local color picker data in several places

if the local color picker is positioned outside the module's image area it can not
deliver valid data. take appropriate action in this case (depends on where we use
the color picker).
blending: add option to toggle polarity of drawn mask

this is needed to effectively extend a parametric mask by including or
excluding specific parts of an image.

to include parts: switch combobox combine to "inclusive" and make sure
polarities of drawn mask and parametric mask channels are set to negative.

to exclude parts: switch combobox combine to "exclusive" and make sure
polarities of drawn mask and parametric mask channels are set to positive.

use "inclusive & inverted" or "exclusive & inverted" to get an inverted mask
if needed.

the "toggle polarity" button (circle with black&white) can be used to quickly
toggle between inclusive and exclusive mode.