sokol_framebuffer.h: post write-api modernizations

Some code cleanup and modernizatin after the new sokol-gfx write API is complete:

• there should be a write-transient hint for pixels and (separately) palette
• there needs to be a validation check when sfb_render() is called while the pixel or palette image don't have valid content:
  • for write-persistent: when pixel and/or palette data hasn't been updated at all yet
  • for write-transient: when pixel and/or palette data hasn't been updated in the current frame