Simulation GUI: Add 2D field view (#2261)

The field image and robot image can be loaded or just a wireframe used.
The robot can be moved and rotated with a mouse click + drag.
The robot position is settable in robot code via the Field2d class.

simulation/halsim_gui/src/main/native/include/GuiUtil.h

@@ -0,0 +1,29 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 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 <GL/gl3w.h>
+#include <imgui.h>
+#include <wpi/Twine.h>
+
+namespace halsimgui {
+
+bool LoadTextureFromFile(const wpi::Twine& filename, GLuint* out_texture,
+                         int* out_width, int* out_height);
+
+// get distance^2 between two ImVec's
+inline float GetDistSquared(const ImVec2& a, const ImVec2& b) {
+  float deltaX = b.x - a.x;
+  float deltaY = b.y - a.y;
+  return deltaX * deltaX + deltaY * deltaY;
+}
+
+// maximize fit while preserving aspect ratio
+void MaxFit(ImVec2* min, ImVec2* max, float width, float height);
+
+}  // namespace halsimgui

simulation/halsim_gui/src/main/native/include/HALSimGui.h

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* 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.                                                               */
@@ -23,6 +23,7 @@ void HALSIMGUI_SetWindowVisibility(const char* name, int32_t visibility);
 void HALSIMGUI_SetDefaultWindowPos(const char* name, float x, float y);
 void HALSIMGUI_SetDefaultWindowSize(const char* name, float width,
                                     float height);
+void HALSIMGUI_SetWindowPadding(const char* name, float x, float y);
 int HALSIMGUI_AreOutputsDisabled(void);
 
 }  // extern "C"
@@ -127,6 +128,14 @@ class HALSimGui {
    */
   static void SetDefaultWindowSize(const char* name, float width, float height);
 
+  /**
+   * Sets internal padding of window added with AddWindow().
+   * @param name window name (same as name passed to AddWindow())
+   * @param x horizontal padding
+   * @param y vertical padding
+   */
+  static void SetWindowPadding(const char* name, float x, float y);
+
   /**
    * Returns true if outputs are disabled.
    *