platforms/x11: Port away from GLTexture::discard()

Currently, if discard() is called, kwin will crash because
EglPixmapTexture does not override the discard method.

In principle, neither GlxPixmapTexture nor EglPixmapTexture should mess
around with internals of the GLTexture class. It is better to have a
wrapper texture with a bind method, which will re-bind the pixmap to the
opengl texture if necessary.