drawing: parse numbers as floats

Parse values as float and convert into 26.6 fixed point. This actually
simplifies scaling a bit.

Fixes #63.
Merge pull request #66 from grigorig/drawing-floats

drawing: parse numbers as floats
shaper: fix nasty sign bug in font functions

Fixes issue #68.
Don't crash on \fscx0 with non-zero \blur

Regression in commit 98707694.
Remove \be upper limit
Benchmark