Auto merge of #1835 - glennw:tidy-hw, r=emilio

Move hardware composite shader to a single file, to match others.

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Author: bors-servo

webrender/res/ps_hardware_composite.fs.glsl

@@ -6,3 +6,34 @@
 
 varying vec3 vUv;
 flat varying vec4 vUvBounds;
+
+#ifdef WR_VERTEX_SHADER
+void main(void) {
+    CompositeInstance ci = fetch_composite_instance();
+    AlphaBatchTask dest_task = fetch_alpha_batch_task(ci.render_task_index);
+    AlphaBatchTask src_task = fetch_alpha_batch_task(ci.src_task_index);
+
+    vec2 dest_origin = dest_task.render_target_origin -
+                       dest_task.screen_space_origin +
+                       vec2(ci.user_data0, ci.user_data1);
+
+    vec2 local_pos = mix(dest_origin,
+                         dest_origin + src_task.size,
+                         aPosition.xy);
+
+    vec2 texture_size = vec2(textureSize(sCacheRGBA8, 0));
+    vec2 st0 = src_task.render_target_origin;
+    vec2 st1 = src_task.render_target_origin + src_task.size;
+    vUv = vec3(mix(st0, st1, aPosition.xy) / texture_size, src_task.render_target_layer_index);
+    vUvBounds = vec4(st0 + 0.5, st1 - 0.5) / texture_size.xyxy;
+
+    gl_Position = uTransform * vec4(local_pos, ci.z, 1.0);
+}
+#endif
+
+#ifdef WR_FRAGMENT_SHADER
+void main(void) {
+    vec2 uv = clamp(vUv.xy, vUvBounds.xy, vUvBounds.zw);
+    oFragColor = texture(sCacheRGBA8, vec3(uv, vUv.z));
+}
+#endif