Also protect concurrent access to p_sys between main thread and libvlc callbacks

fixes: #4827

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.