Fix font rendering at the top of the viewport

[kkaefer]

When the map is tilted, glyphs at the top of the viewport are shaded incorrectly:

[1ec5]

Since #3135 landed, the new osxapp obscures the problematic labels by default with a translucent toolbar. You can still see the issue if you hide the toolbar though.

I see that this PR has a “do not merge” label on it – what’s left to do?

[jfirebaugh]

@kkaefer What's left here? Can this be merged?

[jfirebaugh]

It looks like one of the two commits was merged in 2700ea8. @kkaefer, if you want the other commit ("only write shaders if the file content changed"), please rebase and merge in a new PR.

[kkaefer]

@jfirebaugh 2700ea8 is no longer necessary, since we're now generating the shaders individually with gyp rules, rather than always writing them.