Move screen size constants from video_core to core

video_core didn't even properly use them, and they were the source of
many otherwise-unnecessary dependencies from core to video_core.

src/core/3ds.h

@@ -0,0 +1,21 @@
+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+namespace Core {
+
+// 3DS Video Constants
+// -------------------
+
+// NOTE: The LCDs actually rotate the image 90 degrees when displaying. Because of that the
+// framebuffers in video memory are stored in column-major order and rendered sideways, causing
+// the widths and heights of the framebuffers read by the LCD to be switched compared to the
+// heights and widths of the screens listed here.
+constexpr int kScreenTopWidth = 400;     ///< 3DS top screen width
+constexpr int kScreenTopHeight = 240;    ///< 3DS top screen height
+constexpr int kScreenBottomWidth = 320;  ///< 3DS bottom screen width
+constexpr int kScreenBottomHeight = 240; ///< 3DS bottom screen height
+
+} // namespace Core