UI Code Quality: Use derived struct for progessbar buttons

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

Part of T74432.

1 files changed, 7 insertions, 1 deletions

diff --git a/source/blender/editors/interface/interface_intern.h b/source/blender/editors/interface/interface_intern.h
index 9840444c0e0..41110883729 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_PROGRESS_BAR: Use to store progress (0..1).
    */
   float a1;
 
@@ -322,6 +321,13 @@ typedef struct uiButDecorator {
   int rnaindex;
 } uiButDecorator;
 
+typedef struct uiButProgressbar {
+  uiBut but;
+
+  /* 0..1 range */
+  float progress;
+} uiButProgressbar;
+
 /**
  * Additional, superimposed icon for a button, invoking an operator.
  */