LibVideo/VP9: Remove magic numbers for the uncompressed ref frames

This also adds a fixme to Symbols.h to group and rename the definitions
in the file.

Userland/Libraries/LibVideo/VP9/Parser.cpp

@@ -221,9 +221,9 @@ DecoderErrorOr<FrameContext> Parser::uncompressed_header()
             frame_size = TRY(parse_frame_size());
             render_size = TRY(parse_render_size(frame_size));
         } else {
-            reference_frames_to_update_flags = TRY_READ(m_bit_stream->read_f8());
-            for (auto i = 0; i < 3; i++) {
-                frame_context.reference_frame_indices[i] = TRY_READ(m_bit_stream->read_bits(3));
+            reference_frames_to_update_flags = TRY_READ(m_bit_stream->read_bits(NUM_REF_FRAMES));
+            for (auto i = 0; i < REFS_PER_FRAME; i++) {
+                frame_context.reference_frame_indices[i] = TRY_READ(m_bit_stream->read_bits(LOG2_OF_NUM_REF_FRAMES));
                 frame_context.reference_frame_sign_biases[ReferenceFrameType::LastFrame + i] = TRY_READ(m_bit_stream->read_bit());
             }
             frame_size = TRY(parse_frame_size_with_refs(frame_context.reference_frame_indices));

Userland/Libraries/LibVideo/VP9/Symbols.h

@@ -9,6 +9,12 @@
 
 namespace Video::VP9 {
 
+// FIXME: These should be placed in logical groupings based on the
+//        context they are used in, and perhaps split into multiple
+//        files. While doing so, as many of these as possible should be
+//        renamed to be more human-readable, and most if not all should
+//        be constexpr variables rather than preprocessor definitions.
+
 #define REFS_PER_FRAME 3
 #define MV_FR_SIZE 4
 #define MVREF_NEIGHBOURS 8
@@ -20,7 +26,8 @@ namespace Video::VP9 {
 #define MIN_TILE_WIDTH_B64 4
 #define MAX_TILE_WIDTH_B64 64
 #define MAX_MV_REF_CANDIDATES 2
-#define NUM_REF_FRAMES 8
+#define LOG2_OF_NUM_REF_FRAMES 3
+#define NUM_REF_FRAMES 1 << LOG2_OF_NUM_REF_FRAMES
 #define MAX_REF_FRAMES 4
 #define IS_INTER_CONTEXTS 4
 #define COMP_MODE_CONTEXTS 5