Added al_{horizontal,vertical}_shear_transform.
ex_video: Fix error message.
ex_video: Use builtin font.
ex_video: Make variables static.
video: Make al_get_video_aspect_ratio return non-positive if unknown.
ogv: Fix crash when unable to open a video.
ogv: Don't scan to end of headers of unknown stream.

For example, a comments stream is unrecognised.
ogv: Stop reading at EOF.
ogv: Try to account for audio buffers in positioning.

This improves A-V sync - the video was slightly earlier than the audio.
video: Make seek_to a method parameter.
video: Make al_seek_video return result.
ogv: Move event queue into OGG_VIDEO structure.
ogv: Implement seeking to beginning (only).