Fix hue-rotate and saturate filter

[demo99]

The result of hue-rotate and saturate was wrong. I also create color matrix in vertex shader for those matrix type filters.

After the fix, the gecko looks good: https://treeherder.mozilla.org/#/jobs?repo=try&revision=ca4753a3913013cdc80bd5cbba81f2213ce9ae51&selectedJob=143243269

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This change is 

[glennw]

Thanks for the PR!

There seem to be several unexpected fails in filter tests in that try run that look related though?

Also, do the tests check for correctly working with pre-multiplied alpha? We should make sure we have some coverage for those too if not already.

[demo99]

Oh, I think there is a bug after I move color matrix to vertex shader. I will fix it and add premultiplied alpha test as well.

[glennw]

Looks good! Do we have / need a gecko try run for the updated PR?

[demo99]

The try result is same:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=191652942d7af413180593f77738a71239d7d98d&selectedJob=143539747

There are 5 unexpected fails. The max difference is only 1~2. So I will use fuzz for those tests.

[glennw]

@bors-servo r+

[bors-servo]

📌 Commit f2909b4 has been approved by glennw

[bors-servo]

⌛ Testing commit f2909b4 with merge 66ddb1f...

[bors-servo]

☀️ Test successful - status-appveyor, status-travis
Approved by: glennw
Pushing 66ddb1f to master...

[staktrace]

@demo99 This seems to have caused shader compilation failures on windows. See https://bugzilla.mozilla.org/show_bug.cgi?id=1415150#c7 - the try push that revealed it is at https://treeherder.mozilla.org/#/jobs?repo=try&revision=ffb402d6fefebf42d40c88889b97a86340fda4fe

[mstange]

There's a variable with the name c at

webrender/webrender/res/ps_blend.glsl

Lines 54 to 57 in 3b85b02

	case 3:
	// HueRotate
	float c = cos(vAmount * 0.01745329251);
	float s = sin(vAmount * 0.01745329251);

Maybe variable declarations aren't allowed within switch case branches without a nested scope, similar to C++?

[kvark]

@mstange that's a good candidate! Filed #2024