aout_FiltersCreatePipeline() deals with it internally.

This reverts vlm callback behaviour before 67b16b1b

It avoids creating/destroying texture uselessly.

It is enabled only on linux/win32 when opengl ES is not used and when
non power of 2 textures are supported.

Only RGBA colorspace is supported for the subpicture.

Not yet used.

It will allow to add subpictures blending.

This should fix cursor (un)hiding with some OpenGL back-ends

 It seems that some intel drivers did not like the syntax used to embed the
coefficients directly into the source code.
 Also, it is more flexible and will allow runtime changes.