GPencil: Fix Assert using fill tool

With the new changes in the Draw Manager, GPU_depth_mask must be set to ON, before clear depth.

source/blender/editors/gpencil/gpencil_fill.c

@@ -454,6 +454,7 @@ static bool gpencil_render_offscreen(tGPDfill *tgpf)
   GPU_matrix_push();
   GPU_matrix_identity_set();
 
+  GPU_depth_mask(true);
   GPU_clear_color(0.0f, 0.0f, 0.0f, 0.0f);
   GPU_clear_depth(1.0f);
 
@@ -467,6 +468,8 @@ static bool gpencil_render_offscreen(tGPDfill *tgpf)
   const float ink[4] = {1.0f, 0.0f, 0.0f, 1.0f};
   gpencil_draw_datablock(tgpf, ink);
 
+  GPU_depth_mask(false);
+
   GPU_matrix_pop_projection();
   GPU_matrix_pop();