LibVideo/VP9: Replace (DCT|ADST)_(DCT_ADST) with struct TransformSet

Those previous constants were only set and used to select the first and
second transforms done by the Decoder class. By turning it into a
struct, we can make the code a bit more legible while keeping those
transform modes the same size as before or smaller.

Userland/Libraries/LibVideo/VP9/Symbols.h

@@ -43,10 +43,6 @@ namespace Video::VP9 {
 #define PARTITION_TYPES 4
 #define TX_SIZES 4
 #define TX_MODES 5
-#define DCT_DCT 0
-#define ADST_DCT 1
-#define DCT_ADST 2
-#define ADST_ADST 3
 #define MB_MODE_COUNT 14
 #define INTRA_MODES 10
 #define INTER_MODES 4