Clip stack collapse path

[kvark]

Based on #581
Implements the 3rd option explained in #581 (comment) as an optional code path, controlled by CLIP_TASK_COLLAPSE, enabled by default.

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This change is 

[glennw]

Reviewed 14 of 14 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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webrender/src/tiling.rs, line 616 at r1 (raw file):

                        let mask_task_index = match ctx.layer_masks_tasks.get(&(task.tile_id, sc_index)) {
                            Some(ref mask_task_id) => render_tasks.get_task_index(mask_task_id, child_pass_index),
                            None => RenderTaskIndex(i32::MAX as usize), // special sentinel value recognized by the shader

This should be OPAQUE_TASK_INDEX I think.

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webrender/src/tiling.rs, line 1750 at r1 (raw file):

    fn build(self, passes: &mut Vec<RenderPass>) {
        //println!("{:#?}", self.main_render_task);

Can be removed

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Comments from Reviewable

[glennw]

@kvark This looks great! Two very minor nits, then it looks good to me.

Before merging, I'd like to just get @mrobinson to make sure the stacking context interface change is OK. Instead of having an overflow rect in the stacking context, the method to push a new stacking context now takes a clip region, which can be a normal rect, but could include an image mask etc. Does this interface change have any implications for the scroll root code?

[bors-servo]

☔ The latest upstream changes (presumably #592) made this pull request unmergeable. Please resolve the merge conflicts.

[kvark]

Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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webrender/src/tiling.rs, line 616 at r1 (raw file):

Previously, glennw (Glenn Watson) wrote…

This should be OPAQUE_TASK_INDEX I think.

That was added when I needed it on multiple occasions, but it was removed later on. Anyway, I re-added it back now :)

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webrender/src/tiling.rs, line 1750 at r1 (raw file):

Previously, glennw (Glenn Watson) wrote…

Can be removed

ok

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Comments from Reviewable

[kvark]

All concerns are addressed, and the PR is rebased now.

[glennw]

@bors-servo r+

[bors-servo]

📌 Commit 03ce061 has been approved by glennw

[bors-servo]

⌛ Testing commit 03ce061 with merge 6ca1329...

[bors-servo]

☀️ Test successful - status-travis