[cairo] BackingStore::incorporateUpdate should use CompositeCopy to support drawsBackground=false in general Cairo ports

[Tomoki Imai]

Created attachment 354641 [details]
Test HTML which doesn't have background color.

I'm trying to implement a transparent background feature in Cairo port, and found that it works only in GTK+ port because of BackingStore::incorporateUpdate.

# How to reproduce

The other cairo ports doesn't have transparent background feature yet, but this is reproducable in GTK+ port by removing platform specific code.

1. Delete GTK+ specific code from BackingStore::incorporateUpdate
- https://trac.webkit.org/browser/webkit/trunk/Source/WebKit/UIProcess/cairo/BackingStoreCairo.cpp?rev=227728#L96
2. Build WebKitGTK+ as usual
3. Run MiniBrowser with custom background color.
- $ Tools/Scripts/run-minibrowser --gtk --bg-color="rgba(0,0,0,0)"
4. Go to webpage which doesn't have background.
- You can use attached HTML file.

# Expected result

The webpage's background becomes transparent.

# Actual result

You see collapsed image because previous image are not cleared. (I'll attach image)

I'll post the patch.

[Tomoki Imai]

Created attachment 354644 [details]
Actual result of the testcase

At first, MiniBrowser opens WebKitGTK+ webpage. 
After that, I opened Attachment 354641 [details].

[Tomoki Imai]

Created attachment 354645 [details]
Use CompositeCopy to update bitmap to overwrite existing surface

We should set graphicsContext.setCompositeOperation(WebCore::CompositeCopy) before updating surface.
We should overwrite existing surface by the source even if it contains non opaque parts.

[Tomoki Imai]

Created attachment 354649 [details]
Use CompositeCopy to update bitmap to overwrite existing surface

I updated ChangeLog as the bugtitle doesn't match to bugzilla, and the style is different from the others.

[Tomoki Imai]

I guess we can get rid of GTK+ specific code from BackingStore::incorporateUpdate if user-defined background color is used in WebProcess side.
https://trac.webkit.org/browser/webkit/trunk/Source/WebKit/WebProcess/WebPage/WebPage.cpp?rev=238057#L2817

[Carlos Garcia Campos]

Comment on attachment 354649 [details]
Use CompositeCopy to update bitmap to overwrite existing surface

Ok.

[WebKit Commit Bot]

Comment on attachment 354649 [details]
Use CompositeCopy to update bitmap to overwrite existing surface

Clearing flags on attachment: 354649

Committed r238379: <https://trac.webkit.org/changeset/238379>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Radar WebKit Bug Importer]

<rdar://problem/46173057>