Upgrade span fillers ('cspan.h'), I suspect they have a needless precision limitation

[visualizersdotnl]

[visualizersdotnl]

OK - after some inspection, it's been 20 years after all: this piece of code is typical 'ported from MMX' legacy. I can do a rewrite of this at some point but for now (TM) it functions.

As soon as I'm using them in different scenarios I'll have to give them a once-over, and perhaps introduce versions that also handle texturing.

At this stage I'm not quite sure what we're going to do visually, and thus need in terms of code.