Add a property to choose blend function for any buffer and/or program

[Gammamad]

Is your feature request related to a problem? Please describe.
Making a shadow.fsh I need a subtractive blend for transparent things.

Describe the solution you'd like
In addition to
blend.<program>.<buffer>=src dst srcA dsta
I would like to have
blend.function.<program>.<buffer>=<add | subtract | other functions available in opengl>.

Describe alternatives you've considered
For now I am using SNORM texture format because a + (-b) is equals to a-b. But this reduces color resolution without also picking 16 bit per channel format. And it's just a workaround any way...