ngl: Optimize underlines in text

Opportunistically use the coloring program for
drawing underlines instead of the color program.
This avoids program changes in the middle of
text.

For the Emoji text scrolling benchmark, this reduces
the program changes per frame from > 1000 to around 100.

1 files changed, 41 insertions, 3 deletions

diff --git a/gsk/ngl/gsknglrenderjob.c b/gsk/ngl/gsknglrenderjob.c
index b1bda44790..69c92ac80f 100644
--- a/gsk/ngl/gsknglrenderjob.c
+++ b/gsk/ngl/gsknglrenderjob.c
@@ -1360,16 +1360,54 @@ blur_node (GskNglRenderJob       *job,
   *max_y = job->offset_y + node->bounds.origin.y + node->bounds.size.height + half_blur_extra;
 }
 
+#define ATLAS_SIZE 512
+
 static inline void
 gsk_ngl_render_job_visit_color_node (GskNglRenderJob     *job,
                                      const GskRenderNode *node)
 {
   guint16 color[4];
+  GskNglProgram *program;
+  GskNglCommandBatch *batch;
 
   rgba_to_half (gsk_color_node_get_color (node), color);
-  gsk_ngl_render_job_begin_draw (job, CHOOSE_PROGRAM (job, color));
-  gsk_ngl_render_job_draw_rect_with_color (job, &node->bounds, color);
-  gsk_ngl_render_job_end_draw (job);
+
+  /* Avoid switching away from the coloring program for
+   * rendering a solid color.
+   */
+  program = CHOOSE_PROGRAM (job, coloring);
+  batch = gsk_ngl_command_queue_get_batch (job->command_queue);
+
+  if (batch->any.kind == GSK_NGL_COMMAND_KIND_DRAW &&
+      batch->any.program == program->id)
+    {
+      GskNglRenderOffscreen offscreen = {0};
+
+      gsk_ngl_render_job_begin_draw (job, program);
+
+      /* The top left few pixels in our atlases are always
+       * solid white, so we can use it here, without
+       * having to choose any particular atlas texture.
+       */
+      offscreen.was_offscreen = FALSE;
+      offscreen.area.x = 1.f / ATLAS_SIZE;
+      offscreen.area.y = 1.f / ATLAS_SIZE;
+      offscreen.area.x2 = 2.f / ATLAS_SIZE;
+      offscreen.area.y2 = 2.f / ATLAS_SIZE;
+
+      gsk_ngl_render_job_draw_offscreen_with_color (job,
+                                                    &node->bounds,
+                                                    &offscreen,
+                                                    color);
+
+      gsk_ngl_render_job_end_draw (job);
+    }
+  else
+    {
+      gsk_ngl_render_job_begin_draw (job, CHOOSE_PROGRAM (job, color));
+      gsk_ngl_render_job_draw_rect_with_color (job, &node->bounds, color);
+      gsk_ngl_render_job_end_draw (job);
+    }
 }
 
 static inline void