[RFC weston 13/16] compositor: Add a function to test if images transformed by a matrix should be bilinearly filtered

[Bill Spitzak]

Actually the third column is irrelevant if you know the source is a flat 
plane at z=0, which is what the wayland buffers are.

So only 9 numbers in the matrix need to be examined:

  |sx 0  ? tx|
  |0  sy ? ty|
  |?  ?  ?  ?|
  |0  0  ?  1|

or

  |0  sx ? tx|
  |sy 0  ? ty|
  |?  ?  ?  ?|
  |0  0  ?  1|

sx and sy must be ±1, tx and ty must be integers.