UI Code Quality: Store curve-button gradient type in derived struct

We want to move away from using `uiBut.a1`/`a2`, which is a hard to
reason about design.

Part of T74432.

Also correct comment.

1 files changed, 3 insertions, 3 deletions

diff --git a/source/blender/editors/interface/interface_draw.c b/source/blender/editors/interface/interface_draw.c
index 13dd4ce3001..4d53018cd32 100644
--- a/source/blender/editors/interface/interface_draw.c
+++ b/source/blender/editors/interface/interface_draw.c
@@ -1922,7 +1922,7 @@ void ui_draw_but_CURVE(ARegion *region, uiBut *but, const uiWidgetColors *wcol,
               BLI_rcti_size_y(&scissor_new));
 
   /* Do this first to not mess imm context */
-  if (but->a1 == UI_GRAD_H) {
+  if (but_cumap->gradient_type == UI_GRAD_H) {
     /* magic trigger for curve backgrounds */
     const float col[3] = {0.0f, 0.0f, 0.0f}; /* dummy arg */
 
@@ -1945,7 +1945,7 @@ void ui_draw_but_CURVE(ARegion *region, uiBut *but, const uiWidgetColors *wcol,
   /* backdrop */
   float color_backdrop[4] = {0, 0, 0, 1};
 
-  if (but->a1 == UI_GRAD_H) {
+  if (but_cumap->gradient_type == UI_GRAD_H) {
     /* grid, hsv uses different grid */
     GPU_blend(GPU_BLEND_ALPHA);
     ARRAY_SET_ITEMS(color_backdrop, 0, 0, 0, 48.0 / 255.0);
@@ -2002,7 +2002,7 @@ void ui_draw_but_CURVE(ARegion *region, uiBut *but, const uiWidgetColors *wcol,
 
   if (cumap->flag & CUMA_DRAW_SAMPLE) {
     immBegin(GPU_PRIM_LINES, 2); /* will draw one of the following 3 lines */
-    if (but->a1 == UI_GRAD_H) {
+    if (but_cumap->gradient_type == UI_GRAD_H) {
       float tsample[3];
       float hsv[3];
       linearrgb_to_srgb_v3_v3(tsample, cumap->sample);