Nicer video scaling with bilinear filtering

Dear Joker developers,

This pull request aims at improving the video quality by enabling bilinear filtering for the image scaling.

Currently, the video is scaled with a nearest-neighbour filter. This produces 'blocky' images on screen. By switching to bilinear filtering (which is standard), much nicer images are produced.

The change to bilinear scaling implies an additional modification to specify how the filter handles the pixels at the edge of the video. The scaling by default uses a `repeat' mode, which is only appropriate when drawing a repeating pattern. So we use `clamp to edge' as appropriate here for all textures apart from the strip
background.

In one VideoTest (deinterlaceTest), the bilinear filtering is disabled to avoid having to change the test images.

Thanks again for your attention !