various: encapsulate all code in the olive namespace to avoid name collisions

Large-scale code cleanup. Also adds a license to the top of all files that were
missing it.

app/render/pixelformat.h

@@ -28,6 +28,8 @@
 
 #include "render/rendermodes.h"
 
+OLIVE_NAMESPACE_ENTER
+
 class Frame;
 using FramePtr = std::shared_ptr<Frame>;
 
@@ -139,4 +141,6 @@ private:
 
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // BITDEPTHS_H