gl: fix textures size/alignment for semi-planar chromas

There were 2 errors that canceled each others:

 - The proper size for UV textures (GL_RG or GL_LUMINANCE_ALPHA) is w/2 x
   h/2 (and not w/2 x h/4).

 - Fix row length with GL_UNPACK_ROW_LENGTH or via an intermediate buffer.
   Don't use the pixel_pitch but the visible_pitch/texture_width ratio. Indeed,
   the pixel pitch for the UV plan of a semi-planar picture is 1 when the ratio
   is 2.