LibVideo/VP9: Index inter-frame references with named fields or an enum

Previously, we were using size_t, often coerced from bool or u8, to
index reference pairs. Now, they must either be taken directly from
named fields or indexed using the `ReferenceIndex` enum with options
`primary` and `secondary`. With a more explicit method of indexing
these, the compiler can aid in using reference pairs correctly, and
fuzzers may be able to detect undefined behavior more easily.

Userland/Libraries/LibVideo/VP9/Enums.h

@@ -66,6 +66,11 @@ enum ReferenceMode : u8 {
     ReferenceModeSelect = 2,
 };
 
+enum class ReferenceIndex : u8 {
+    Primary = 0,
+    Secondary = 1,
+};
+
 enum BlockSubsize : u8 {
     Block_4x4 = 0,
     Block_4x8 = 1,