don't clear screen during CA_LOADING and CA_PRIMED

Just as it currently doesn't clear for CA_ACTIVE, we also should not be clearing for CA_LOADING and CA_PRIMED (in other words, anytime CG_DrawActiveFrame is called). During these states, trap_UpdateScreen may be called to perform an explicit partial flush, and we shouldn't clear the existing screen when this happens.

It perhaps makes more sense to add a boolean to SCR_UpdateScreen that indicates if this is an explicit partial flush or not (and only clear if not), but I wanted to be conservative with what was changed.

This issue currently manifests itself when loading levels with Q3F with a widescreen resolution:

`+set fs_game q3f2 +set r_mode -1 +set r_customWidth 1024 +set r_customHeight 512 +map q3f_2stag2`

Vanilla Q3 calls trap_UpdateScreen throughout CG_Init, which will clear the screen, and re-draw the loading screen as part of CL_CGameRendering.

Q3F is a bit different. It draws its UI menus throughout CG_Init and calls trap_UpdateScreen to flush the changes, but calling CL_CGameRendering while loading doesn't actually cause anything to be drawn. This results in the UI being drawn over by the screen clear, and only a black screen being rendered during level load.