swrast: use Map/UnmapTextureImage() in framebuffer map/unmap code

swrast: use Map/UnmapTextureImage() in framebuffer map/unmap code

When we're actually rendering into a texture, map the texture image
instead of the corresponding renderbuffer.  Before, we just copied
a pointer from the texture image to the renderbuffer.  This change
will make the code usable by hardware drivers.
(cherry picked from commit 1caf698191fb871850311353862eb7fc927f9f9c)