Fixes https://github.com/googlefonts/fontmake/issues/558
When drawing a composite glyph with a scaled component using the TTGlyphPen, the bounding
box coordinates may change depending on whether one computes them *before* compiling or
*after* decompiling. Before compiling, component.transform holds double precision floats,
but after compiling and decompiling, these are necessarily clamped to F2Dot14 fixed precision.
The TTGlyphPen needs to quantize transform floats to F2Dot14 so that the values don't
change after compilation.
Without this change, it may happen that round-tripping fonts through ttx (which by default
recalcBBoxes) will produce different bounding boxes for composite glyphs that have
scaled or transformed components.
