Remove virtual readPixel() function from RenderDevice

This function is only used by SDLSoftwareRenderDevice, so it doesn't
need to be declared for all render devices.
Remove dstrect parameter from fillWithColor()

All of our uses for this function were passing NULL to this variable in
order to fill the entire image surface.
Merge pull request #1224 from dorkster/renderer

Remove virtual readPixel() function from RenderDevice