static/index.html: Simplify coordinate system handling

Beforehand, the texture loaded from the image was flipped in the Y axis;
to compensate for this, multiple unholy coordinate flips were done, let's
eschew that.
static/index.html lightfield shader: Proper rendering by fixing matrix multiplication order
static/index.html: Keep ofs_{U,V} in the OpenGL coordinate system too
static/index.html: Trigger finer motion by mouse dragging
static/index.html: Show U,V values in the control panel