No perspective interpolation for blend brushes

[kvark]

A follow-up to #2422

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This change is 

[glennw]

Needs a gecko try, and possibly a reference test image update, since there might be a couple of subpixel differences. Otherwise, looks great!

[glennw]

Paraphrasing IRC conversation - for later reference:

Once we support rasterizing pictures in screen or local space, what we'll want to do is:

• Remove the FORCE_NO_PERSPECTIVE define.
• Have brush.glsl select whether to force w=1 based on the raster mode of the picture in question.

[glennw]

Yup, very slight difference in one of the reftests which will need an updated reference image.

[kvark]

Reftest is updated. Also, pending try push: https://treeherder.mozilla.org/#/jobs?repo=try&revision=1f1d9f2620554952095d20d77edd1b3a1bb78301

[kvark]

@staktrace

builds/worker/workspace/build/tests/reftest/tests/dom/html/reftests/bug917595-pixel-rotated.jpg

Looks like an intermittent bug that is also affected by the way we interpolate things.
Is this the case where Gecko updates a reftest, or the PR needs to be fixed (I don't particularly see anything wrong with it, for the record)?

[staktrace]

@kvark you can ignore that one, it was introduced by #2408 and I already have a fuzzing patch in my queue for it.

[kvark]

Thanks!
@bors-servo r=glennw

[bors-servo]

📌 Commit 3a17a77 has been approved by glennw

[bors-servo]

⌛ Testing commit 3a17a77 with merge 114cd5e...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster, status-travis
Approved by: glennw
Pushing 114cd5e to master...