mesa: fix proxy texture target initialization

mesa: fix proxy texture target initialization

The mapping from TEXTURE_x_INDEX to GL_TEXTURE_x was broken in
alloc_proxy_textures() because the elements in the targets[] array
were in the wrong order.

This didn't actually cause any failures since we never really use the
proxy texture's Target field.  But let's get it right.

NOTE: This is a candidate for the 8.0 branch.