LibVideo/VP9: Move persistent context storage to a different header

Moving these to another header allows Parser.h to include less context
structs/classes that were previously in Context.h.

This change will also allow consolidating some common calculations into
Context.h, since we won't be polluting the VP9 namespace as much. There
are quite a few duplicate calculations for block size, transform size,
number of horizontal and vertical sub-blocks per block, all of which
could be moved to Context.h to allow for code deduplication and more
semantic code where those calculations are needed.

Userland/Libraries/LibVideo/VP9/Parser.cpp

@@ -9,6 +9,7 @@
 #include <LibGfx/Point.h>
 #include <LibGfx/Size.h>
 
+#include "Context.h"
 #include "Decoder.h"
 #include "Parser.h"
 #include "Utilities.h"