opengl: use main shader for direct samplers

A sampler may receive their input picture either:
 - from an interop, uploading a picture_t
 - from a previous filter, giving an OpenGL texture directly (a "direct"
   sampler)

The fragment shader for a "direct" sampler was straightforward: the
input was always an RGBA texture. On the contrary, samplers from interop
always applied chroma conversion to expose individual pixels in RGBA.

However, deinterlace filters operate on individual input planes, so
the chroma conversion must be applied afterwards.

To prepare support for this feature, use the same fragment shader code
generation for both types of samplers.

Signed-off-by: Alexandre Janniaux <ajanni@videolabs.io>