Fix #129907: Grease Pencil brush size gets stuck at 0

Possibly due to c13cde24cc

This commit clamps the `brush->size` value to 1 at lowest for the
Grease Pencil Draw mode. There are situations where when the brush scene
space value is set to a small enough size that the distance calculated
by `project_brush_radius` becomes 0.

When this value is set as the actual brush size, the `wm.radial_control`
operator fails to work properly as the new size is now lower than the
expected minimum value, causing incorrect clamping of the modal value.

Pull Request: https://projects.blender.org/blender/blender/pulls/129937

source/blender/editors/sculpt_paint/paint_cursor.cc

@@ -1569,7 +1569,7 @@ static void grease_pencil_brush_cursor_draw(PaintCursorContext *pcontext)
         const float3 location = placement.project(float2(pcontext->x, pcontext->y));
         pcontext->pixel_radius = project_brush_radius(
             &pcontext->vc, brush->unprojected_radius, location);
-        brush->size = pcontext->pixel_radius;
+        brush->size = std::max(pcontext->pixel_radius, 1);
       }
       else {
         pcontext->pixel_radius = brush->size;