GPv3: Hide the paint cursor while drawing

This matches the behavior of GPv2. While drawing a stroke
the paint cursor is not shown.

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -848,8 +848,15 @@ class GreasePencilRuntime {
    * Allocated and freed by the drawing code. See `DRW_grease_pencil_batch_cache_*` functions.
    */
   void *batch_cache = nullptr;
-  /* The frame on which the object was evaluated (only valid for evaluated object). */
+  /**
+   * The frame on which the object was evaluated (only valid for evaluated object).
+   */
   int eval_frame = 0;
+  /**
+   * Set to true while drawing a stroke (e.g. with the draw tool).
+   * Used for example to temporarily hide the paint cursor in the viewport.
+   */
+  bool is_drawing_stroke = false;
 
  public:
   GreasePencilRuntime() {}

source/blender/editors/sculpt_paint/grease_pencil_paint.cc

@@ -719,6 +719,9 @@ void PaintOperation::on_stroke_begin(const bContext &C, const InputSample &start
       CTX_data_main(&C), object, brush);
   const int material_index = BKE_object_material_index_get(object, material);
 
+  /* We're now starting to draw. */
+  grease_pencil->runtime->is_drawing_stroke = true;
+
   PaintOperationExecutor executor{C};
   executor.process_start_sample(*this, C, start_sample, material_index);
 
@@ -949,6 +952,9 @@ void PaintOperation::on_stroke_done(const bContext &C)
   drawing.set_texture_matrices({texture_space_}, IndexRange::from_single(active_curve));
   drawing.tag_topology_changed();
 
+  /* Now we're done drawing. */
+  grease_pencil.runtime->is_drawing_stroke = false;
+
   DEG_id_tag_update(&grease_pencil.id, ID_RECALC_GEOMETRY);
   WM_event_add_notifier(&C, NC_GEOM | ND_DATA, &grease_pencil.id);
 }

source/blender/editors/sculpt_paint/paint_cursor.cc

@@ -1540,6 +1540,11 @@ static void grease_pencil_brush_cursor_draw(PaintCursorContext *pcontext)
       return;
     }
 
+    /* Hide the cursor while drwaing. */
+    if (grease_pencil->runtime->is_drawing_stroke) {
+      return;
+    }
+
     if ((brush->flag & BRUSH_LOCK_SIZE) != 0) {
       const bke::greasepencil::Layer *layer = grease_pencil->get_active_layer();
       const ed::greasepencil::DrawingPlacement placement(