Even if we replace all glClearColor to use alpha 0.0 as default alpha we don't get alpha stored in the RGBA buffer.
If you take a look at the first file you can see that alpha is always
1.0 in BGE (1_alpha_picker.blend)
Once that's working 2Dfilter can be tackled too (2_2D_filter_alpha_tests).
According to Brecht:
"Blender doesn't request the alpha buffer on all OSes"
"if you render to FBO it's a matter of making sure the texture has an alpha channel, if you render into back buffer it requires changes in GHOST"
"also not sure if all cards support it, probably they do, but i don't think it's a requirement for OpenGL"