gpu: Handle alpha in image_op() wrapper

There are various places where the alpha is implicitly assumed to be
handled, so just handle it.

As a bonus, this simplifies a bunch of code and makes the texture node
rendering work with alpha.

gsk/gpu/gskgpunodeprocessor.c

@@ -679,12 +679,24 @@ gsk_gpu_node_processor_image_op (GskGpuNodeProcessor   *self,
     {
       gsk_gpu_straight_alpha_op (self->frame,
                                  gsk_gpu_clip_get_shader_clip (&self->clip, &self->offset, rect),
+                                 self->opacity,
                                  self->desc,
                                  descriptor,
                                  rect,
                                  &self->offset,
                                  tex_rect);
     }
+  else if (self->opacity < 1.0)
+    {
+      gsk_gpu_color_matrix_op_opacity (self->frame,
+                                       gsk_gpu_clip_get_shader_clip (&self->clip, &self->offset, rect),
+                                       self->desc,
+                                       descriptor,
+                                       rect,
+                                       &self->offset,
+                                       tex_rect,
+                                       self->opacity);
+    }
   else
     {
       gsk_gpu_texture_op (self->frame,
@@ -3561,7 +3573,7 @@ static const struct
   },
   [GSK_TEXTURE_NODE] = {
     0,
-    0,
+    GSK_GPU_HANDLE_OPACITY,
     gsk_gpu_node_processor_add_texture_node,
     gsk_gpu_node_processor_create_texture_pattern,
   },

gsk/gpu/gskgpustraightalphaop.c

@@ -54,6 +54,7 @@ static const GskGpuShaderOpClass GSK_GPU_STRAIGHT_ALPHA_OP_CLASS = {
 void
 gsk_gpu_straight_alpha_op (GskGpuFrame            *frame,
                            GskGpuShaderClip        clip,
+                           float                   opacity,
                            GskGpuDescriptors      *desc,
                            guint32                 descriptor,
                            const graphene_rect_t  *rect,
@@ -71,4 +72,5 @@ gsk_gpu_straight_alpha_op (GskGpuFrame            *frame,
   gsk_gpu_rect_to_float (rect, offset, instance->rect);
   gsk_gpu_rect_to_float (tex_rect, offset, instance->tex_rect);
   instance->tex_id = descriptor;
+  instance->opacity = opacity;
 }

gsk/gpu/gskgpustraightalphaopprivate.h

@@ -8,6 +8,7 @@ G_BEGIN_DECLS
 
 void                    gsk_gpu_straight_alpha_op                       (GskGpuFrame                    *frame,
                                                                          GskGpuShaderClip                clip,
+                                                                         float                           opacity,
                                                                          GskGpuDescriptors              *desc,
                                                                          guint32                         descriptor,
                                                                          const graphene_rect_t          *rect,

gsk/gpu/shaders/gskgpustraightalpha.glsl

@@ -4,7 +4,7 @@ PASS(0) vec2 _pos;
 PASS_FLAT(1) Rect _rect;
 PASS(2) vec2 _tex_coord;
 PASS_FLAT(3) uint _tex_id;
-
+PASS_FLAT(4) float _opacity;
 
 
 #ifdef GSK_VERTEX_SHADER
@@ -12,6 +12,7 @@ PASS_FLAT(3) uint _tex_id;
 IN(0) vec4 in_rect;
 IN(1) vec4 in_tex_rect;
 IN(2) uint in_tex_id;
+IN(3) float in_opacity;
 
 void
 run (out vec2 pos)
@@ -24,6 +25,7 @@ run (out vec2 pos)
   _rect = r;
   _tex_coord = rect_get_coord (rect_from_gsk (in_tex_rect), pos);
   _tex_id = in_tex_id;
+  _opacity = in_opacity;
 }
 
 #endif
@@ -37,7 +39,7 @@ run (out vec4 color,
      out vec2 position)
 {
   color = gsk_texture_straight_alpha (_tex_id, _tex_coord) *
-          rect_coverage (_rect, _pos);
+          rect_coverage (_rect, _pos) * _opacity;
   position = _pos;
 }