vector: Fix overlapping glyphs in shields

The new shield style uses images in formatted text, with a text-rotation-alignment of viewport-glyph, to render shields. viewport-glyph aligned symbols weren't subject to the line simplification that map-aligned line-placed symbols were. The line simplification function also handles splitting lines at sharp angles. So in some cases, a viewport-glyph symbol was placed along a sharp angle, resulting in overlapping images.

Fixed by also simplifying lines for viewport-glyph symbols.