…hs in local space. r=lsalzman

This patch makes the CPU side incorporate the raster scale when
calculating the subpixel position of a glyph. It also makes the shader
side not include the glyph scale factor when recalculating the glyph
position (since it was not known/used when determining the subpixel
position in the first place). This makes the subpixel position stable
when we transition between Screen and Local(raster_scale) spaces.

Differential Revision: https://phabricator.services.mozilla.com/D62812

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/9bd4120bc9779fe1c42fb45246044a23f2e95d25