Allow for non opaque colors for src-over lcd fallback case in gpu

Since skia is currently requiring the dst to be opaque for lcd draws this
change is safe to make in terms of correctly. It will also be faster than
the current shader based blending.

Bug: chromium:725879
Change-Id: I0c14b80eb5ab819500afc55030df83d5ddbef94a
Reviewed-on: https://skia-review.googlesource.com/17840
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 834f3af..9012d20 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -801,7 +801,8 @@
     }
 
     if (isLCD) {
-        if (SkBlendMode::kSrcOver == mode && color.isConstant() && color.isOpaque() &&
+        // See comment in MakeSrcOverXferProcessor about color.isOpaque here
+        if (SkBlendMode::kSrcOver == mode && color.isConstant() && /*color.isOpaque() &&*/
             !caps.shaderCaps()->dualSourceBlendingSupport() &&
             !caps.shaderCaps()->dstReadInShaderSupport()) {
             props |= AnalysisProperties::kIgnoresInputColor;
@@ -896,7 +897,15 @@
         return nullptr;
     }
 
-    if (color.isConstant() && color.isOpaque() && !caps.shaderCaps()->dualSourceBlendingSupport() &&
+    // Currently up the stack Skia is requiring that the dst is opaque or that the client has said
+    // the opaqueness doesn't matter. Thus for src-over we don't need to worry about the src color
+    // being opaque or not. For now we disable the check for opaqueness, but in the future we should
+    // pass down the knowledge about dst opaqueness and make the correct decision here.
+    //
+    // This also fixes a chrome bug on macs where we are getting random fuzziness when doing
+    // blending in the shader for non opaque sources.
+    if (color.isConstant() && /*color.isOpaque() &&*/
+        !caps.shaderCaps()->dualSourceBlendingSupport() &&
         !caps.shaderCaps()->dstReadInShaderSupport()) {
         // If we don't have dual source blending or in shader dst reads, we fall
         // back to this trick for rendering SrcOver LCD text instead of doing a