LibVideo/VP9: Implement a bit stream to decode VP9 data

The VP9 specification requires a special decoding process to parse a
lot of the data read from a frame, so this BitStream wrapper implements
that behavior. These processes are defined in section 9 of the
VP9 spec.

Userland/Libraries/LibVideo/VP9/BitStream.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2021, Hunter Salyer <thefalsehonesty@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/Types.h>
+
+namespace Video::VP9 {
+
+class BitStream {
+public:
+    BitStream(const u8* data, size_t size)
+        : m_data_ptr(data)
+        , m_bytes_remaining(size)
+    {
+    }
+
+    u8 read_byte();
+    bool read_bit();
+    u8 read_f(size_t n);
+    i8 read_s(size_t n);
+    u8 read_f8();
+    u16 read_f16();
+    u8 read_literal(size_t n);
+
+    u64 get_position();
+    size_t bytes_remaining();
+    size_t bits_remaining();
+
+    bool init_bool(size_t bytes);
+    bool read_bool(u8 probability);
+    bool exit_bool();
+
+private:
+    const u8* m_data_ptr { nullptr };
+    size_t m_bytes_remaining { 0 };
+    Optional<u8> m_current_byte;
+    i8 m_current_bit_position { 0 };
+    u64 m_bytes_read { 0 };
+
+    u8 m_bool_value { 0 };
+    u8 m_bool_range { 0 };
+    u64 m_bool_max_bits { 0 };
+};
+
+}