Cleanup: GPv3: Add drawing API for fill color

This adds two functions to the `Drawing` class: `fill_colors()`
and `fill_colors_for_write()`.
In places where the attribute API was used directly, these
functions are now used instead.

This is part of #119080. The point is that developers
shouldn't need to remember the name, type, default value,
and domain of built-in attributes (e.g. used by the renderer).

@@ -240,3 +240,3 @@
 static void modify_fill_color(Object &ob,
                               const GreasePencilTintModifierData &tmd,
-                              bke::CurvesGeometry &curves,
+                              Drawing &drawing,