SkPDF: with opaque draws, treat SRC mode as SRC_OVER (cherrypick a681433 and 3098a75)

SkPDF: with opaque draws, treat SRC mode as SRC_OVER (cherrypick a681433 and 3098a75)

Also: Better comments in SkXfermodeInterpretation

Review URL: https://codereview.chromium.org/1159493003
Review URL: https://codereview.chromium.org/1159763004

BUG=473572
NOTREECHECKS=true
NOTRY=true
R=reed@google.com

Committed: https://skia.googlesource.com/skia/+/cc124276a29275f09c7ee294e60ad7fcd9ca317a

[hal.canary, 2015-06-08 16:41:27]

halcanary@google.com changed reviewers:
+ reed@google.com

[hal.canary, 2015-06-08 16:41:28]

they asked for this to be cherry-picked.

[reed1, 2015-06-08 17:30:09]

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
File src/core/SkXfermodeInterpretation.cpp (right):

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
src/core/SkXfermodeInterpretation.cpp:10: 
Does this function need to return true only if there is a single color, or can
it return true if the "src" is opaque? Perhaps it should have a diff name.

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
src/core/SkXfermodeInterpretation.cpp:11: static bool just_solid_color(const
SkPaint& p) {
We can inspect shaders and colorfilters as well to catch more cases.

[hal.canary, 2015-06-08 17:40:37]

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
File src/core/SkXfermodeInterpretation.cpp (right):

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
src/core/SkXfermodeInterpretation.cpp:10: 
On 2015/06/08 17:30:09, reed1 wrote:
> Does this function need to return true only if there is a single color, or can
> it return true if the "src" is opaque? Perhaps it should have a diff name.

Good question, but outside the scope of this CL.  Notice that this code is
logically the same as the code I removed (with the same name) from SkBlitter. 
For a cherry-pick, I don't want to change the SkBlitter functionality at all.

https://codereview.chromium.org/1160623007/diff/1/src/core/SkXfermodeInterpre...
src/core/SkXfermodeInterpretation.cpp:11: static bool just_solid_color(const
SkPaint& p) {
On 2015/06/08 17:30:09, reed1 wrote:
> We can inspect shaders and colorfilters as well to catch more cases.

same.

[reed1, 2015-06-08 17:44:31]

lgtm

[hal.canary, 2015-06-08 17:48:58]

Committed patchset #1 (id:1) manually as
cc124276a29275f09c7ee294e60ad7fcd9ca317a (presubmit successful).