opengl: add blend deinterlace filter

--deinterlace-mode selects a blend algorithm, but not a particular implementation.

--deinterlace-mode=blend

or equivalently ("deinterlace" is the default filter):

--deinterlace-filter=deinterlace --deinterlace-mode=blend

To run the OpenGL version (the mode is "auto"):

--deinterlace-filter=glblend

The module glblend only implements the "blend" algorithm, it can be passed explicitly:

--deinterlace-filter=glblend --deinterlace-mode=blend

To pass a list a modules, executed in order until one works:

--deinterlace-filter=glblend,deinterlace --deinterlace-mode=blend