vo_opengl: refactor: merge convolution function and sampler entrypoint

There's not much of a reason to have the actual convolution code in a
separate function. Merging them actually simplifies the code a bit, and
gets rid of the repetitious macro invocations to define the functions
for each filter size.

There should be no changes in behavior or output.
vo_opengl: never use 1D textures for lookup textures

This was a microoptimization for small filters which need 4 or less
weights per sample point. When I originally wrote this code, using a 1D
texture seemed to give a slight speed gain, but now I couldn't measure
any difference.

Remove this to simplify the code.
vo_opengl: refactor: instantiate scaler functions at runtime

Before this commit, the convolution scaler shader functions were pre-
instantiated in the shader file. For every filter size, a corresponding
function (with the filter size as suffix) had to be present.

Change this, and make the C code emit the necessary bits.

This means the shader code is much reduced. (Although hopefully it
doesn't make shader compilation faster - it would require a really dumb
compiler if it spends its time on dead code.)

It also makes it more flexible, which is the main goal.

The DEF_SCALER0 stuff is needed because the C code writes the header of
the shader, at a point where scaler macros are not defined yet.
vo_opengl: use all filter sizes possible with the shaders

Not all filter sizes the shaders could handle were in the filter_sizes
list. The shader can handle any multiple of 4 (the sizes 2 and 6 are
special-cased to keep it simple).

Add all possible filter sizes, up to 64. 64 is ridiculously high anyway.
Most of the larger filter sizes are completely useless for upscaling,
but help with the fancy-downscaling option. (Although it would still be
more efficient to use cascaded scalers to handle downscaling better.)

I considered doing something less stupid than the hardcoded array, but
it seems this is still the simplest solution.
vo_opengl: enable fancy-downscaling for opengl-hq again

I guess most problems with it have been fixed.

It's still slower than necessary, though.