Forget the name of a texture after freeing

Bug #6408362

FontRenderer allocates large font textures when more room is needed
to store all the glyphs used by an application. Thse large textures
are the first to be freed when memory needs to be reclaimed by the
system. When freeing a texture, the renderer would however not set
the texture name to an invalid name, leading future allocations to
be performed on the same texture name. That name could have by then
be recycled by the driver and returned by a call to glGenTexture
and used to create an entirely different texture. This would cause
the font renderer to point to the wrong texture, thus leading to
the "corruptions."

Change-Id: I8a1e80e5b79e8f21d1baf5320c090df4f2066cd4