[wpigui] Add wpigui wrappers for GLFW+imgui

These hide the platform specifics behind a common C++ API.  Platforms:
 - Windows: DirectX 11 (with 10 backwards compatibility)
 - Linux: OpenGL 3
 - Mac: Metal

wpigui/src/main/native/include/wpigui_internal.h

@@ -0,0 +1,73 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <atomic>
+#include <functional>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include <GLFW/glfw3.h>
+#include <imgui.h>
+
+namespace wpi::gui {
+
+struct SavedSettings {
+  bool loadedWidthHeight = false;
+  int width;
+  int height;
+  int maximized = 0;
+  int xPos = -1;
+  int yPos = -1;
+  int userScale = 2;
+  int style = 0;
+};
+
+struct Font {
+  static constexpr int kScaledLevels = 9;
+  ImFont* scaled[kScaledLevels];
+};
+
+struct Context : public SavedSettings {
+  std::atomic_bool exit{false};
+
+  std::string title;
+  int defaultWidth;
+  int defaultHeight;
+
+  GLFWwindow* window = nullptr;
+
+  std::vector<std::function<void()>> initializers;
+  std::vector<std::function<void(float scale)>> windowScalers;
+  std::vector<std::pair<
+      const char*,
+      std::function<ImFont*(ImGuiIO& io, float size, const ImFontConfig* cfg)>>>
+      makeFonts;
+
+  ImVec4 clearColor = ImVec4(0.45f, 0.55f, 0.60f, 1.00f);
+  std::vector<std::function<void()>> earlyExecutors;
+  std::vector<std::function<void()>> lateExecutors;
+
+  int fontScale = 2;  // updated by main loop
+  std::vector<Font> fonts;
+};
+
+extern Context* gContext;
+
+void PlatformCreateContext();
+void PlatformDestroyContext();
+void PlatformGlfwInitHints();
+void PlatformGlfwWindowHints();
+bool PlatformInitRenderer();
+void PlatformRenderFrame();
+void PlatformShutdown();
+
+void CommonRenderFrame();
+
+}  // namespace wpi::gui