lua.wetgenes.grd.blit
g:blit(gb,x,y,cx,cy,cw,ch)
Blit a 2D area from one grd into another.
gb is the grd to blit from.
x,y is the location to blit too.
cx,cy,cw,ch is a clip area that should be applied to gb before it is blitted. EG to specify an area within gb. If not provided it will default to the entirety of gb,
g (destination) must be FMT_U8_RGBA and gb (source) must be FMT_U8_RGBA_PREMULT this function will blend the images using normal alpha blending.
This is not overly optimised but should be reasonably fast.