speed up the pyramid gradient using memcpy's. also make it not crash for 1px high...

speed up the pyramid gradient using memcpy's.  also make it not crash for 1px high textures.

here are some sample profiling results.  pyramid2 is the new code

  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name

 58.78      1.54     1.54      255     6.04     6.04  gradient_pyramid1
 40.46      2.60     1.06      255     4.16     4.16  gradient_pyramid2

 54.88      2.25     2.25      504     4.46     4.46  gradient_pyramid1
 44.88      4.09     1.84      504     3.65     3.65  gradient_pyramid2