Set the alpha per-surface.

A surface with enabled alpha blending(per-pixel or per-surface) will be never created on video memory if alpha blending is not supported by the hardware. Right now, only glSDL supports it.
With per-pixel alpha, per-surface alpha is useless, so any attempt to activate it will be ignored.

Note that enabling and disabling the alpha blending on the execution time could slowdown your graphic performance.
To extend the information about the alpha blending,