Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/GPU2D_Soft.cpp

@@ -30,7 +30,7 @@ SoftRenderer::SoftRenderer(melonDS::GPU& gpu)
     // mosaic table is initialized at compile-time
 }
 
-u32 SoftRenderer::ColorComposite(int i, u32 val1, u32 val2)
+u32 SoftRenderer::ColorComposite(int i, u32 val1, u32 val2) const
 {
     u32 coloreffect = 0;
     u32 eva, evb;