Add a few texture manipulation tools

[Exter-N]

• Add a constant row to the color transformation matrix, allowing for example to express per-channel inversions ;
• Add a few matrix presets (identity, grayscale, extract channel) ;
• Replace the "Invert Colors" checkbox by a button that does the same thing at the matrix level ;
• Add "Invert Red", "Invert Green", "Invert Blue", "Invert Alpha" buttons ;
• Add a "Save in place" button, that replaces the input (left) texture by the combined texture (this button is grayed out if the combined texture is an unchanged copy of the input, or if the input is a game path) ;
• Merge the "Save as TEX", "Save as DDS" and "Save as PNG" buttons into a single one that picks the format according to the extension of the destination file ;
• Let the in-place conversion buttons stay visible if they don't apply, graying them out instead, for layout consistency ;
• Also gray them out if the input is a game path ;
• Add several choices of combine operations to the overlay texture:
  • Over (standard composition, as already existed) ;
  • Under (standard composition, but in reverse) ;
  • Ignore (don't take the overlay into account) ;
  • Replace (only take the overlay into account, enabling import flows where the destination texture is selected on the left, the source is selected on the right, and "Save in place" does the job) ;
  • Copy channels (useful for importing into Multi maps, in conjunction with the extract matrix presets for exporting), with options for the channels that are to be taken from the left or from the right.