Fix for 4B colour channel on screen capture

 * Geist only supports Windows with 32 bit colour depth
 * Set the bitmap bmiHeader.biBitCount to 32
 * Create numpy array as h x w x 4B
 * Strip out alpha channel and swap bgr to rgb
Merge pull request #4 from ant1441/alpha-channel-bugfix

Fix for 4B colour channel on screen capture