Don't round drawing positions or sizes to integers.

Enables correct subpixel rendering/movement on native platforms, which is supported by Lime, and solves the problems that arise when rounding all positions (i.e. http://forum.haxepunk.com/index.php?topic=540.0) These small rounding errors can be very noticeable for small objects. Appearance on native and Flash is more consistent this way.

This also fixes an issue rendering Tilemaps with an alpha less than 1. Sometimes the right edge of one tile and the left edge of another would be rounded to the same position and overlap, and that edge would be drawn over twice. When alpha is less than 1, this results in noticeable seams.