Fix #147168: BLF: Glitchy status bar display

This seems to be a Mesa driver error.

Discarding the buffer before the area drawing fixes the
issue.

Given this is an expensive operation, we try to only do it
where it is needed.

Pull Request: https://projects.blender.org/blender/blender/pulls/147733

source/blender/blenfont/BLF_api.hh

@@ -175,6 +175,10 @@ void BLF_batch_draw_begin();
 void BLF_batch_draw_flush();
 void BLF_batch_draw_end();
 
+/* Discard any batching in process and restart.
+ * Only used as a workaround for glitchy driver sync. */
+void BLF_batch_discard();
+
 /**
  * Draw the string using the current font.
  */

source/blender/blenfont/intern/blf_font.cc

@@ -343,6 +343,15 @@ static void blf_batch_draw_end()
   }
 }
 
+void BLF_batch_discard()
+{
+  if (g_batch.glyph_buf) {
+    GPU_storagebuf_free(g_batch.glyph_buf);
+  }
+  g_batch.glyph_buf = GPU_storagebuf_create_ex(
+      sizeof(g_batch.glyph_data), nullptr, GPU_USAGE_STREAM, __func__);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/editors/screen/area.cc

@@ -46,6 +46,7 @@
 #include "GPU_immediate.hh"
 #include "GPU_immediate_util.hh"
 #include "GPU_matrix.hh"
+#include "GPU_platform.hh"
 #include "GPU_state.hh"
 
 #include "BLF_api.hh"
@@ -3902,6 +3903,13 @@ void ED_region_header_draw(const bContext *C, ARegion *region)
 {
   /* clear */
   ED_region_clear(C, region, region_background_color_id(C, region));
+
+  if (GPU_type_matches_ex(GPU_DEVICE_ANY, GPU_OS_UNIX, GPU_DRIVER_OPENSOURCE, GPU_BACKEND_OPENGL))
+  {
+    /* WORKAROUND: Driver bug. Fixes invalid glyph being rendered (see #147168). */
+    BLF_batch_discard();
+  }
+
   region_draw_blocks_in_view2d(C, region);
   ED_region_draw_overflow_indication(CTX_wm_area(C), region);
 }