Blender Git Loki

Camera tracking: use texture buffers (if supported) to display clip editor frames

Use texture buffers to display frames of footage in clip editor. This allows
to apply bilinear filtering of proxied resolution which.

This also resolves incredibly slow performance when drawing 4K footage on
some videocards (was originally noticed on macbook pro). Also this allows
to avoid sending the whole frame to the video memory when working with a
single frame (i.e. before this patch the whole frame would be send to the
videocard when panning frame).