LibVideo: Combine VP9's Intra- and InterMode enums into PredictionMode

The two different mode sets are stored in single fields, and the
underlying values didn't overlap, so there was no reason to keep them
separate.

The enum is now an enum class as well, to enforce that almost all uses
of the enum are named. The only case where underlying values are used
is in lookup tables, but it may be worth abstracting that as well to
make array bounds more clear.

Userland/Libraries/LibVideo/VP9/LookupTables.h

@@ -107,15 +107,15 @@ static constexpr int partition_tree[6] = {
 static constexpr int cols_partition_tree[2] = { -PartitionHorizontal, -PartitionSplit };
 static constexpr int rows_partition_tree[2] = { -PartitionVertical, -PartitionSplit };
 static constexpr int intra_mode_tree[18] = {
-    -DcPred, 2,
-    -TmPred, 4,
-    -VPred, 6,
+    -to_underlying(PredictionMode::DcPred), 2,
+    -to_underlying(PredictionMode::TmPred), 4,
+    -to_underlying(PredictionMode::VPred), 6,
     8, 12,
-    -HPred, 10,
-    -D135Pred, -D117Pred,
-    -D45Pred, 14,
-    -D63Pred, 16,
-    -D153Pred, -D207Pred
+    -to_underlying(PredictionMode::HPred), 10,
+    -to_underlying(PredictionMode::D135Pred), -to_underlying(PredictionMode::D117Pred),
+    -to_underlying(PredictionMode::D45Pred), 14,
+    -to_underlying(PredictionMode::D63Pred), 16,
+    -to_underlying(PredictionMode::D153Pred), -to_underlying(PredictionMode::D207Pred)
 };
 static constexpr int segment_tree[14] = {
     2, 4, 6, 8, 10, 12,
@@ -133,9 +133,9 @@ static constexpr int tx_size_16_tree[4] = {
 };
 static constexpr int tx_size_8_tree[2] = { -TX_4x4, -TX_8x8 };
 static constexpr int inter_mode_tree[6] = {
-    -(ZeroMv - NearestMv), 2,
-    -(NearestMv - NearestMv), 4,
-    -(NearMv - NearestMv), -(NewMv - NearestMv)
+    -to_underlying(PredictionMode::ZeroMv), 2,
+    -to_underlying(PredictionMode::NearestMv), 4,
+    -to_underlying(PredictionMode::NearMv), -to_underlying(PredictionMode::NewMv)
 };
 static constexpr int interp_filter_tree[4] = {
     -EightTap, 2,