Limit the number of used vertex attributes to 16

[kvark]

Some platforms (e.g. Vulkan on Intel) have a limit of 16 vertex attributes that we have to respect. Currently, cs_border_segment shader uses 17 attributes. We need to either combine some of them into fewer ones, or avoid including some of the unused ones, plus put up an assertion to not exceed the limit again. Also see szeged#180

[kvark]

Apparently, we are doing good here. It was a problem with Szeged fork.