am 35b38cef: Improve drawing cache speed by selecting the correct opacity and keeping a 32 bits format when the window is 32 bits.

Merge commit '35b38cefcc92f1ed599a652ac5736ab9e9e75039' into eclair-mr2

* commit '35b38cefcc92f1ed599a652ac5736ab9e9e75039':
  Improve drawing cache speed by selecting the correct opacity and keeping a 32 bits