gallium: Set renderbuffer's InternalFormat when rendering to texture

gallium: Set renderbuffer's InternalFormat when rendering to texture

When an FBO is rendering to a texture (rather than a renderbuffer),
Gallium sets up an internal renderbuffer to handle the rendering, and
copies over enough texture state to make this work.

InternalFormat was missed out, causing glTexCopyImage to take a slow
path unnecessarily.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=41263

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Brian Paul <brianp@vmware.com>