min/max for 64 bit scatterplot

[yocontra]

Opening to make sure it doesn't get lost in the void.

I could use a little help with the PR for the 64 bit layer. The 32 bit was easy with clamp(), the 64 bit layer seems much more complex and I'm not sure where to dive in.

Might be easier if @shaojingli tackles this instead of waiting for me to fumble a PR out.

[howtimeflies0]

Issue addressed in #283