mesa: fix _mesa_get_fallback_texture() to handle all texture targets

mesa: fix _mesa_get_fallback_texture() to handle all texture targets

Previously, this function only handled 2D textures.

The fallback texture is used when we try to sample from an incomplete
texture object.  GLSL says sampling an incomplete texture should return
(0,0,0,1).

v2: use a 1-texel texture image, per José.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>