mesa: Fixed a texture memory leak

mesa: Fixed a texture memory leak

The current texture for any particular texture unit is given an additional
reference in update_texture_state(); but if the context is closed before
that texture can be released (which is quite frequent in normal use, unless
a program unbinds and deletes the texture and renders without it to force
a call to update_texture_state(), the memory is lost.

This affects general Mesa; but the i965 is particularly affected because
it allocates a considerable amount of additional memory for each allocated
texture.

(cherry picked from master, commit c230767d6956b63a2b101acb48f98823bb5dd31a)