gpu: Optimize box-shadow shader

Like in the border shader, don't draw the (potentially large for the
window's shadow) inside part that is transparent.

gsk/gpu/gskgpuboxshadowop.c

@@ -37,6 +37,24 @@ gsk_gpu_box_shadow_op_print (GskGpuOp    *op,
   gsk_gpu_print_newline (string);
 }
 
+#ifdef GDK_RENDERING_VULKAN
+static GskGpuOp *
+gsk_gpu_box_shadow_op_vk_command (GskGpuOp              *op,
+                                  GskGpuFrame           *frame,
+                                  GskVulkanCommandState *state)
+{
+  return gsk_gpu_shader_op_vk_command_n (op, frame, state, 8);
+}
+#endif
+
+static GskGpuOp *
+gsk_gpu_box_shadow_op_gl_command (GskGpuOp          *op,
+                                  GskGpuFrame       *frame,
+                                  GskGLCommandState *state)
+{
+  return gsk_gpu_shader_op_gl_command_n (op, frame, state, 8);
+}
+
 static const GskGpuShaderOpClass GSK_GPU_BOX_SHADOW_OP_CLASS = {
   {
     GSK_GPU_OP_SIZE (GskGpuBoxShadowOp),
@@ -44,9 +62,9 @@ static const GskGpuShaderOpClass GSK_GPU_BOX_SHADOW_OP_CLASS = {
     gsk_gpu_shader_op_finish,
     gsk_gpu_box_shadow_op_print,
 #ifdef GDK_RENDERING_VULKAN
-    gsk_gpu_shader_op_vk_command,
+    gsk_gpu_box_shadow_op_vk_command,
 #endif
-    gsk_gpu_shader_op_gl_command
+    gsk_gpu_box_shadow_op_gl_command
   },
   "gskgpuboxshadow",
   sizeof (GskGpuBoxshadowInstance),

gsk/gpu/shaders/gskgpuboxshadow.glsl

@@ -19,20 +19,37 @@ IN(6) float in_shadow_spread;
 IN(7) float in_blur_radius;
 IN(8) uint in_inset;
 
+#define GAUSSIAN_SCALE_FACTOR ((3.0 * sqrt(2.0 * PI) / 4.0))
+
 void
 run (out vec2 pos)
 {
+  Rect bounds = rect_from_gsk (in_bounds);
+  RoundedRect outside = rounded_rect_from_rect (bounds);
   RoundedRect outline = rounded_rect_from_gsk (in_outline);
+  vec2 spread = GSK_GLOBAL_SCALE * in_shadow_spread;
   
-  pos = rect_get_position (rect_from_gsk (in_bounds));
+  _clip_outline = outline;
+
+  RoundedRect inside;
+  if (in_inset == 0u)
+    {
+      inside = outline;
+      spread = -spread;
+      outline = rounded_rect_shrink (outline, spread.yxyx);
+      rounded_rect_offset (outline, GSK_GLOBAL_SCALE * in_shadow_offset);
+    }
+  else
+    {
+      outline = rounded_rect_shrink (outline, spread.yxyx);
+      rounded_rect_offset (outline, GSK_GLOBAL_SCALE * in_shadow_offset);
+      inside = outline;
+      inside = rounded_rect_shrink (inside, (in_blur_radius * GAUSSIAN_SCALE_FACTOR * GSK_GLOBAL_SCALE).yxyx);
+    }
+
+  pos = border_get_position (outside, inside);
 
   _pos = pos;
-  _clip_outline = outline;
-  vec2 spread = GSK_GLOBAL_SCALE * in_shadow_spread;
-  if (in_inset == 0u)
-    spread = -spread;
-  outline = rounded_rect_shrink (outline, spread.yxyx);
-  rounded_rect_offset (outline, GSK_GLOBAL_SCALE * in_shadow_offset);
   _shadow_outline = outline;
   _color = in_color;
   _sigma = GSK_GLOBAL_SCALE * 0.5 * in_blur_radius;

gsk/gpu/shaders/roundedrect.glsl

@@ -11,6 +11,12 @@ struct RoundedRect
   vec4 corner_heights;
 };
 
+RoundedRect
+rounded_rect_from_rect (Rect r)
+{
+  return RoundedRect (r.bounds, vec4 (0.0), vec4 (0.0));
+}
+
 RoundedRect
 rounded_rect_from_gsk (mat3x4 gsk_rounded_rect)
 {