GPU queries refactor

[kvark]

Removes query compile feature. Fixes #1994
Refactors the GPU queries quite a bit, including moving the logic into a dedicated module.
Ideally should be tested on Android, but we can patch it afterwards anyway.

Note: the API is asking for some type-level protection against:

• doing anything outside of frame bounds
• starting to sample queries in a row
  But I don't see a way to make it stronger without introducing obscure programming patterns :) So I'm leaving it with just start/finish semantics, plus a bit of RAII.

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This change is 

[glennw]

I didn't get to this today, sorry - will do first thing in the morning. It looks great so far from a quick scan though!

CI failures:

$ if [ $BUILD_KIND = DEBUG ]; then (cd webrender && cargo build --verbose --features profiler,query); fi

error: Package `webrender v0.53.2 (file:///home/travis/build/servo/webrender/webrender)` does not have these features: `query`

The command "if [ $BUILD_KIND = DEBUG ]; then (cd webrender && cargo build --verbose --features profiler,query); fi" exited with 101.

[glennw]

(We probably need to update both the travis.yml and the taskcluster config)

[glennw]

Reviewed 12 of 14 files at r1.
Review status: 12 of 14 files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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webrender/src/profiler.rs, line 882 at r1 (raw file):

        );

        if false {

Should we remove this or comment it perhaps?

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Comments from Reviewable

[glennw]

(Partial review of the easy bits so far - will cover the remaining files in the morning).

[glennw]

Reviewed 2 of 14 files at r1, 3 of 3 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks broke.

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Comments from Reviewable

[glennw]

Looks good, thanks!

[glennw]

I noticed that when I toggle the query feature on and off, a small box occurs in the top left of the screen - it looks like perhaps the quad for the profile counters is still being drawn?

Not a big deal anyway - we can fix that as a follow up if you like.

So, r=me - I'll leave it to you to squash / fix that bug if you want to, or merge now :)

[kvark]

Thanks @glennw ! Fix appeared to be rather trivial: 19731c1#diff-59bdb33ab120f3af70ac6ca84fd5451fR882

@bors-servo r=glennw

[bors-servo]

📌 Commit 19731c1 has been approved by glennw

[bors-servo]

⌛ Testing commit 19731c1 with merge 0f95ff0...

[bors-servo]

☀️ Test successful - status-appveyor, status-travis
Approved by: glennw
Pushing 0f95ff0 to master...

[staktrace]

@kvark This caused webrender to crash on windows, see https://bugzilla.mozilla.org/show_bug.cgi?id=1417062#c5