Adjust transparency handling in mix/overlay + more renderer tests + no OoB in SIMD transparency handling

Author: Stovent

src/CDI/Video/Renderer.hpp

@@ -273,7 +273,7 @@ class Renderer
 constexpr Pixel Renderer::backdropCursorColorToPixel(const uint8_t color) noexcept
 {
     // Background plane has no transparency (Green book V.5.13).
-    Pixel argb = 0xFF'00'00'00; // Set transparency for cursor plane.
+    Pixel argb = 0xFF'00'00'00; // Background and cursor color pixels are always visible.
     const uint8_t c = bit<3>(color) ? Renderer::PIXEL_FULL_INTENSITY : Renderer::PIXEL_HALF_INTENSITY;
     if(bit<2>(color)) argb.r = c; // Red.
     if(bit<1>(color)) argb.g = c; // Green.