gallium: fix some render to texture bugs

Before, we were sometimes rendering into a stale texture because
st_finalize_texture() would discard the old texture and create a new one.

Moved st_update_framebuffer atom after texture validation so that we
can create a new renderbuffer surface if the texture changes.

Also, split texture validation into two parts: finalize_textures and
update_textures.  Do finalize_textures first to avoid getting into the
situtation where we're doing a pipe->surface_copy() mid-way through
state validation.

Some debug code still in place, but disabled...

src/mesa/state_tracker/st_cb_readpixels.c

@@ -183,6 +183,8 @@ st_readpixels(GLcontext *ctx, GLint x, GLint y, GLsizei width, GLsizei height,
    struct gl_pixelstore_attrib clippedPacking = *pack;
    struct pipe_surface *surf;
 
+   assert(ctx->ReadBuffer->Width > 0);
+
    /* XXX convolution not done yet */
    assert((transferOps & IMAGE_CONVOLUTION_BIT) == 0);