Replace FALLTHROUGH macro by C++17 [[fallthrough]]

This attribute is now part of the standard we target so we no longer
need compiler-specific hacks.

Also enables -Wimplicit-fallthrough for Clang now that we can properly
support it. It's already on by default for GCC's -Wextra.

Fixes new warnings raised by Clang's -Wimplicit-fallthrough.

servers/visual/rasterizer.h

@@ -1080,7 +1080,7 @@ public:
 						const Item::CommandTransform *transform = static_cast<const Item::CommandTransform *>(c);
 						xf = transform->xform;
 						found_xform = true;
-						FALLTHROUGH;
+						[[fallthrough]];
 					}
 					default: {
 						c = c->next;