Add input and output stride parameters.

This allows a subset of the screen to be taken as a source for upscaling, as well as a subset of the screen to be taken as a destination for downscaling, if the stride is set properly for both cases.

In the future, these will allow framebuffers to have different strides for different outputs: LCD, TV-out and HDMI.
Merge pull request #5 from Nebuleon/master

Add input and output stride parameters.