Use of secondary texture in image shader (renamed from mask texture) in image filters for ichannel_1 as texture of display loopback only if not needed otherwise (e.g. input image needed in Sharpen filter).
Enable use of a source as mask for another source. Improved Mask mechanism in Source class, with various flags for update of source (avoid repeated mask update (GPU costly). Using SourceLink to link source to mask (improved robustness of SourceLink).