This fixes a rendering issue (and ultimately a crash) in PowerPoint when
compiling with GCC 8.

GCC8 doesn't support the `excess-precision=standard` option under the
`#pragma GCC optimize` directive.

This results in unpredictable floating point rounding leading to errors
when inserting segments (with missing edges and/or triangles).

Using 24-bit precision ensures we don't have any excess precision.