st/nine: Remove Managed texture hack.

st/nine: Remove Managed texture hack.

Previously binding an unitialized managed texture
was causing a crash, and a workaround was added to
prevent the crash.

This patch removes this workaround and instead set the initial
state of managed textures as dirty, so that when the texture is bound
for the first time, it is always initialized.

Signed-off-by: Axel Davy <axel.davy@ens.fr>