d2d1: Use 24-bit FP precision for triangulate.
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.