Rendering lots of dots/dashes

[nical]

WebRender currently generates per-dot and per-dash data is sent to the GPU (as segment data I think). In some unfortunate cases we can end up allocating hundreds of thousands of them which isn't working out very well.

One way we could go about this is to have dashes and dots drawn on the GPU given some initial information computed on the CPU about how many there are and whatever is needed to place them properly (which according to the amount of effort that goes into it in gecko doesn't appear to be that simple).

[kvark]

There are 2 problems here:

1. the amount of data we may want to send via GPU cache. It's unlikely going to change per frame, so from the performance standpoint it's only a single spike, but it may theoretically hit the limit of the GPU cache vertical size.
2. the fact we expect all the segments from a primitive to fit inside a single GPU cache line, which is very limited (i.e. 512 segments). This could be alleviated by splitting a primitives into sub-primitives, so that each has segments fitting into their corresponding GPU cache lines.

[gw3583]

With the way borders are drawn and cached now, we don't (as far as I know) store this information in the GPU cache. @mrobinson and I changed it to store the dot / dash data inside the instance, so that we don't hit GPU cache size limits.

[nical]

Ah I see. We still run into cases where we have so many dots and dashes that the CPU side can freeze for a second pushing hundreds of thousands of dots into a vector, though.