LibVideo/VP9: Use an enum to select segment features

This throws out some ugly `#define`s we had that were taking the role
of an enum anyway. We now have some nice getters in the contexts that
take the place of the combo of `seg_feature_active()` and then doing a
lookup in `FrameContext::m_segmentation_features` directly.

Userland/Libraries/LibVideo/VP9/Enums.h

@@ -161,4 +161,17 @@ enum Token : u8 {
     DctValCat6 = 10,
 };
 
+enum class SegmentFeature : u8 {
+    // SEG_LVL_ALT_Q
+    UseAlternativeQuantizerBase,
+    // SEG_LVL_ALT_L
+    UseAlternativeLoopFilterBase,
+    // SEG_LVL_REF_FRAME
+    ReferenceFrameOverride,
+    // SEG_LVL_SKIP
+    SkipResidualsOverride,
+    // SEG_LVL_MAX
+    Sentinel,
+};
+
 }