UI Code Quality: Use derived struct for color buttons

For the main rationale behind this design, see 03b122e2a18df. Further,
this removes users of `uiBut.a1`/`uiBut.a2`, which is a very ugly design
choice (hard to reason about).

Part of Part of T74432.

1 files changed, 8 insertions, 2 deletions

diff --git a/source/blender/editors/interface/interface_intern.h b/source/blender/editors/interface/interface_intern.h
index 0dfa23a4332..9840444c0e0 100644
--- a/source/blender/editors/interface/interface_intern.h
+++ b/source/blender/editors/interface/interface_intern.h
@@ -177,7 +177,6 @@ struct uiBut {
    * - UI_BTYPE_LABEL:        Use `(a1 == 1.0f)` to use a2 as a blending factor (imaginative!).
    * - UI_BTYPE_SCROLL:       Use as scroll size.
    * - UI_BTYPE_SEARCH_MENU:  Use as number or rows.
-   * - UI_BTYPE_COLOR:        Use as indication of color palette.
    * - UI_BTYPE_PROGRESS_BAR: Use to store progress (0..1).
    */
   float a1;
@@ -188,7 +187,6 @@ struct uiBut {
    * - UI_BTYPE_NUM:          Use to store RNA 'precision' value, for dragging and click-step.
    * - UI_BTYPE_LABEL:        If `(a1 == 1.0f)` use a2 as a blending factor.
    * - UI_BTYPE_SEARCH_MENU:  Use as number or columns.
-   * - UI_BTYPE_COLOR:        Use as index in palette (not so good, needs refactor).
    */
   float a2;
 
@@ -281,6 +279,14 @@ struct uiBut {
   uiBlock *block;
 };
 
+/** Derived struct for #UI_BTYPE_COLOR */
+typedef struct uiButColor {
+  uiBut but;
+
+  bool is_pallete_color;
+  int palette_color_index;
+} uiButColor;
+
 /** Derived struct for #UI_BTYPE_TAB */
 typedef struct uiButTab {
   uiBut but;