Bug fixes related to resizing.

Fixes buttons not being cleared after resizing with some GuiScreens.

Fixes screenshots being rather messed up on some resolutions, i.e. where width % 4 did not equal 0. (also fixes crashing on taking screenshots on some graphics card)

Screenshot below shows both of these bugs in action:
![screenshot_20140317200047](https://f.cloud.github.com/assets/6509348/2457921/647627cc-af42-11e3-8b70-f4142adffa95.png)