GRIM/EMI: Fix sprite scaling with non-native screen resolutions

When mapping a sprite to a GL polygon its dimensions are scaled by the ratio
between the current screen resolution and the native game resolution. When
the polygon is rendered it is scaled again, causing sprites to be displayed
unproportionally big when using larger screen resolutions.
Merge pull request #808 from inguin/sprite-scaling

GRIM/EMI: Fix sprite scaling with non-native screen resolutions