video_core: Resolve more variable shadowing scenarios

Resolves variable shadowing scenarios up to the end of the OpenGL code
to make it nicer to review. The rest will be resolved in a following
commit.

src/video_core/command_classes/codecs/vp9.cpp

@@ -233,7 +233,7 @@ constexpr std::array<s32, 254> map_lut{
 }
 } // Anonymous namespace
 
-VP9::VP9(GPU& gpu) : gpu(gpu) {}
+VP9::VP9(GPU& gpu_) : gpu{gpu_} {}
 
 VP9::~VP9() = default;
 

src/video_core/command_classes/codecs/vp9.h

@@ -108,7 +108,7 @@ private:
 
 class VP9 {
 public:
-    explicit VP9(GPU& gpu);
+    explicit VP9(GPU& gpu_);
     ~VP9();
 
     VP9(const VP9&) = delete;