Refactor the script task to conform to current coding style

drawFocusIfNeeded draws a focus ring

Vertex attributes cleanup

This is the preparation work for servo#457

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Instanced attributes

Closes servo#457

Performance-wise, I wasn't able to register a noticeable difference. Tested on https://github.com/servo/servo, full screen after the second page down, on `Mesa DRI Intel(R) HD Graphics 5500 (Broadwell GT2)` with resolution 2560x1440.

With the change, I got 4.5 ms mean GPU time in the first test, and 4.3 ms time in the second.
Without the change, I got 4.4 ms mean GPU time in the only test.

I suppose the difference is not visible since we are far from being VS-bound.

Note: this does not replace the optimization of having one large buffer in servo#456. Instanced attributes will benefit from it in the same way as UBOs.

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