cogl-clip-stack: Don't convert rect clips to window clips

cogl-clip-stack: Don't convert rect clips to window clips

Previously in cogl-clip-state.c when it detected that the current
modelview matrix is screen-aligned it would convert the clip entry to
a window clip. Instead of doing this cogl-clip-stack.c now contains
the detection and keeps the entry as a rectangle clip but marks that
it is entirely described by its scissor rect. When flusing the clip
stack it doesn't do anything extra for entries that have this mark
(because the clip will already been setup by the scissor). This is
needed so that we can still track the original rectangle coordinates
and modelview matrix to help detect when it would be faster to modify
the rectangle when adding it to the journal rather than having to
break up the batch to set the clip state.