JANITORIAL: Silence some GCC 7 warnings

We have lots and lots of -Wimplicit-fallthrough warnings, and I
don't know if it's worth fixing them or not. But if we want to,
this is how it can be done.

engines/agi/global.cpp

@@ -69,7 +69,7 @@ byte AgiEngine::getVar(int16 varNr) {
 	switch (varNr) {
 	case VM_VAR_SECONDS:
 		getVarSecondsHeuristicTrigger();
-		// is supposed to fall through
+		// fall through
 	case VM_VAR_MINUTES:
 	case VM_VAR_HOURS:
 	case VM_VAR_DAYS:

engines/agi/graphics.cpp

@@ -1003,7 +1003,7 @@ void GfxMgr::drawBox(int16 x, int16 y, int16 width, int16 height, byte backgroun
 	case Common::kRenderHercA:
 	case Common::kRenderHercG:
 		lineColor = 0; // change linecolor to black
-		// supposed to fall through
+		// fall through
 	case Common::kRenderCGA:
 	case Common::kRenderEGA:
 	case Common::kRenderVGA:
@@ -1027,7 +1027,7 @@ void GfxMgr::drawDisplayRect(int16 x, int16 y, int16 width, int16 height, byte c
 	case Common::kRenderHercA:
 		if (color)
 			color = 1; // change any color except black to green/amber
-		// supposed to fall through
+		// fall through
 	case Common::kRenderEGA:
 	default:
 		drawDisplayRectEGA(x, y, width, height, color);

engines/sci/engine/kstring.cpp

@@ -374,6 +374,7 @@ reg_t kFormat(EngineState *s, int argc, reg_t *argv) {
 			case 'x':
 			case 'u':
 				unsignedVar = true;
+				/* fall through */
 			case 'd': { /* Copy decimal */
 				/* int templen; -- unused atm */
 				const char *format_string = "%d";