Add st3m_gfx_fb_save/restore and sys_display.fb_save/restore

These can be used to temporarily store a screenshot of a framebuffer, which can be useful to switch out from an application into (for example) the help menu, draw into the normal fb instead of the osd fb, and then switch back to the application allowing for manual redraws.

Calling save twice drops the old saved buffer. Calling restore twice makes the second call a no-op.