pixelsampler: added pixel sampler panel

When visible, this panel will show the current reference and display transformed
color under the cursor of any viewer.

app/widget/viewer/viewerglwidget.h

@@ -24,8 +24,10 @@
 #include <QOpenGLWidget>
 
 #include "render/backend/opengl/openglcolorprocessor.h"
+#include "render/backend/opengl/openglframebuffer.h"
 #include "render/backend/opengl/openglshader.h"
 #include "render/backend/opengl/opengltexture.h"
+#include "render/color.h"
 #include "render/colormanager.h"
 
 /**
@@ -119,15 +121,37 @@ public slots:
    */
   void SetMatrix(const QMatrix4x4& mat);
 
+  /**
+   * @brief Enables or disables whether this color at the cursor should be emitted
+   *
+   * Since tracking the mouse every movement, reading pixels, and doing color transforms are processor intensive, we
+   * have an option for it. Ideally, this should be connected to a PixelSamplerPanel::visibilityChanged signal so that
+   * it can automatically be enabled when the user is pixel sampling and disabled for optimization when they're not.
+   */
+  void SetSignalCursorColorEnabled(bool e);
+
 signals:
+  /**
+   * @brief Signal emitted when the user starts dragging from the viewer
+   */
   void DragStarted();
 
+  /**
+   * @brief Signal emitted when cursor color is enabled and the user's mouse position changes
+   */
+  void CursorColor(const Color& reference, const Color& display);
+
 protected:
   /**
    * @brief Override the mouse press event simply to emit the DragStarted() signal
    */
   virtual void mousePressEvent(QMouseEvent* event) override;
 
+  /**
+   * @brief Override mouse move to provide functionality for
+   */
+  virtual void mouseMoveEvent(QMouseEvent* event) override;
+
   /**
    * @brief Initialize function to set up the OpenGL context upon its construction
    *
@@ -183,6 +207,11 @@ private:
    */
   OpenGLColorProcessorPtr color_service_;
 
+  /**
+   * @brief Framebuffer used for reading reference space pixels from the texture
+   */
+  OpenGLFramebuffer frame_buffer_;
+
   /**
    * @brief Drawing matrix (defaults to identity)
    */
@@ -199,6 +228,8 @@ private:
 
   bool has_image_;
 
+  bool signal_cursor_color_;
+
 private slots:
   /**
    * @brief Slot to connect just before the OpenGL context is destroyed to clean up resources