CMakeLists: Specify -Wextra on linux builds

Allows reporting more cases where logic errors may exist, such as
implicit fallthrough cases, etc.

We currently ignore unused parameters, since we currently have many
cases where this is intentional (virtual interfaces).

While we're at it, we can also tidy up any existing code that causes
warnings. This also uncovered a few bugs as well.

src/core/frontend/framebuffer_layout.cpp

@@ -25,7 +25,7 @@ FramebufferLayout DefaultFrameLayout(u32 width, u32 height) {
     ASSERT(height > 0);
     // The drawing code needs at least somewhat valid values for both screens
     // so just calculate them both even if the other isn't showing.
-    FramebufferLayout res{width, height};
+    FramebufferLayout res{width, height, false, {}};
 
     const float window_aspect_ratio = static_cast<float>(height) / width;
     const float emulation_aspect_ratio = EmulationAspectRatio(