implement alpha/opacity for areas

[mk30]

No description provided.

[mk30]

current plan:

make 3 new draw calls - pointtransp, linetransp, areatransp.probably shader will be same, but depth might be enable/disable, mask enable/disable..not sure.

canvas api - read pixel values as rgba from the texture. loop over them. separate out into 2 buffers: everything that's fully opaque and anything with some transparency. (as you go over everything in the data buffer, you can access the type. use that to know where to look in the style.png for each zoom level.)

sort transparency buffer by zindex value. pass the sorted transparent bucket as props into the transparency draw call.

benchmark canvas program vs js program. benchmark should run in browser not in node.

[mk30]

points and lines now complete. there's a single "line-opacity" setting. in the future, this can be split out to separate "line-fill-opacity" and "line-stroke-opacity" settings.

next step: areas.