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.

The effect is hard to notice in Grim where the only sprites seem to be puffs of
smoke, but it's obvious in EMI which makes heavy use of sprites.