Fix accessBitmap() with deferred clear, GPU path.

Fix accessBitmap() with deferred clear, GPU path.

When accessBitmap() is called on a GPU device which has deferred clear,
and no drawing has been done, force the clear before returning the
bitmap.

This is covered by the top right test case in the imagefilterscropexpand
GM, where an offset image filter is drawn but the resulting bitmap is
cropped out, resulting in no drawing to the offscreen. It should be
empty, but without this fix it results in garbage (previous scratch
texture contents).

BUG=skia:
R=bsalomon@google.com

Committed: https://code.google.com/p/skia/source/detail?r=13860

[Stephen White, 2014-03-19 18:09:20]

Brian: PTAL. Thanks!

[bsalomon, 2014-03-19 18:15:21]

Nice find. LGTM.

[Stephen White, 2014-03-19 18:24:08]

Committed patchset #1 manually as r13860 (presubmit successful).

[reed1, 2014-03-19 18:26:44]

please please please tell me we have a plan to make all of this work w/o
"accessBitmap" on the device....?

Can we start coding with this "new" pattern now, to make it easier for me to
deprecated the existing
bitmaps-are-really-sometimes-textures-that-sometimes-call-readpixels-behind-your-back
?

[Stephen White, 2014-03-19 18:30:01]

On 2014/03/19 18:26:44, reed1 wrote:
> please please please tell me we have a plan to make all of this work w/o
> "accessBitmap" on the device....?
> 
> Can we start coding with this "new" pattern now, to make it easier for me to
> deprecated the existing
>
bitmaps-are-really-sometimes-textures-that-sometimes-call-readpixels-behind-your-back
> ?

Pretty much all the "generic" filters (that use straight-up Skia calls) have a
"*result = accessBitmap(false);" call at the end to return the backing store as
a bitmap. They're all modeled on the SkTestImageFilters that you wrote. :)

Switching to SkSurface/SkImage would fix this, presumably.

[reed1, 2014-03-19 18:53:09]

Definitely the ones I wrote are now using the wrong pattern. We need something,
either SkSurface, or some new abstraction created just for imagefilter-pipeline,
to do this.