gpu, cmaa: optimize COMBINE_EDGES path to reduce fragment shader tasks

gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc

Index: gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc b/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
index 65eca9ad36afcc8233508bcb22a70cae8970dcdb..671b2565ce31e0b6e932121154309e3ffbcfe25d 100644
--- a/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
+++ b/gpu/command_buffer/service/gles2_cmd_apply_framebuffer_attachment_cmaa_intel.cc
@@ -399,9 +399,7 @@ void ApplyFramebufferAttachmentCMAAINTELResourceManager::ApplyCMAAEffectTexture(
   }
 
   //  - Combine RightBottom (.xy) edges from previous pass into
-  //    RightBottomLeftTop (.xyzw) edges and output it into the mask (have to
-  //    fill in the whole buffer including empty ones for the line length
-  //    detection to work correctly).

[dshwang, 2016/07/06 10:27:30]

It's wrong, because all edge information is generated by DETECT_EDGES1 path. All
pixel which DETECT_EDGES1 didn't mark don't have any information of edges.

+  //    RightBottomLeftTop (.xyzw) edges and output it into the mask.
   //  - On all pixels with any edge, input buffer into a temporary color buffer
   //    needed for correct blending in the next pass (other pixels not needed
   //    so not copied to avoid bandwidth use).
@@ -415,13 +413,13 @@ void ApplyFramebufferAttachmentCMAAINTELResourceManager::ApplyCMAAEffectTexture(
   //  gl_FragDepth                        mini4_edge_texture_          fbo.depth
   {
     // Combine edges: each pixel will now contain info on all (top, right,
-    // bottom, left) edges; also create depth mask as above depth and mark
-    // potential Z sAND also copy source color data but only on edge pixels
+    // bottom, left) edges; also mark depth 1 value on all pixels with any edge
+    // and also copy source color data but only on edge pixels
     glUseProgram(edges_combine_shader_);
-    glUniform1f(0, 1.0f);
+    glUniform1f(0, 0.0f);
     glUniform2f(1, 1.0f / width_, 1.0f / height_);
     glDepthMask(GL_TRUE);
-    glDepthFunc(GL_ALWAYS);
+    glDepthFunc(GL_LESS);
     glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
 
     if (!is_gles31_compatible_) {