Fix #26218: texture paint

- Added option "Fixed Texture" to the UI. Because of strange reason,
  this feature was implemented but hidden from users.
  Would be cool, if somebody familiar with 2d texture paiting check.
- Fixed some issues in existing code of fixed texture paiting.
  It now handles brush radius and curve correct.
- Also fixed issue with paiting with texture from node tree - it used
  to be painted with regular brush color instead of texture.

release/scripts/ui/space_image.py

@@ -685,6 +685,7 @@ class IMAGE_PT_tools_brush_texture(BrushButtonsPanel, bpy.types.Panel):
 
         col = layout.column()
         col.template_ID_preview(brush, "texture", new="texture.new", rows=3, cols=8)
+        col.prop(brush, "use_fixed_texture")
 
 
 class IMAGE_PT_tools_brush_tool(BrushButtonsPanel, bpy.types.Panel):

release/scripts/ui/space_view3d_toolbar.py

@@ -694,6 +694,8 @@ class VIEW3D_PT_tools_brush_texture(PaintPanel, bpy.types.Panel):
         col = layout.column()
 
         col.template_ID_preview(brush, "texture", new="texture.new", rows=3, cols=8)
+        if brush.use_paint_texture:
+            col.prop(brush, "use_fixed_texture")
 
         if context.sculpt_object:
             #XXX duplicated from properties_texture.py