Handle box-shadow stretch mode per-axis.

[glennw]

This is a crash fix for box shadows that have one axis which is
very long, and one axis that is small.

It's also an optimization for box-shadows that have one long and
one short axis, since it allows drawing a minimal shadow rect in
these cases. This is handled by having a stretch mode per-axis
rather than one for the entire shadow primitive.

Fixes #2561.

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This change is 

[glennw]

r? @mrobinson or anyone

Pending try run here (have not checked results yet):
https://treeherder.mozilla.org/#/jobs?repo=try&revision=c9bbe08556ee1d784cbeab6f345adde782cfac72

[glennw]

Try run looks good. There are a couple of seemingly unrelated intermittents, but all the box-shadow reftests appear the same.

[glennw]

r? @kvark

[kvark]

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Reviewed 9 of 9 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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webrender/res/cs_clip_box_shadow.glsl, line 23 at r1 (raw file):

    vec2 src_rect_size;
    float clip_mode;
    int stretch_mode_x;

nit: could be ivec2 stretch_mode;

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webrender/res/cs_clip_box_shadow.glsl, line 77 at r1 (raw file):

    }

    switch (bs_data.stretch_mode_y) {

somewhat unfortunate code duplication, can be addressed later

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Comments from Reviewable

[kvark]

@bors-servo r+

[bors-servo]

📌 Commit b0f31e8 has been approved by kvark

[bors-servo]

⌛ Testing commit b0f31e8 with merge 53750b8...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: kvark
Pushing 53750b8 to master...