r600g: various fixes

r600g: various fixes

- enabled flushing a buffer more than once
- enabled the blitter for r600_clear
- added some more colors to r600_is_format_supported (copied from r600_conv_pipe_format)
- r600_set_framebuffer_state now sets rctx->fb_state
- more states are saved before a blit (had to add some accounting for the viewport and the vertex elements state)
- fixed a few errors with reference counting