implement document resize with menu and dialog

src/rtt.h

@@ -8,6 +8,7 @@ class RTT
     GLuint fboID = 0;
     GLuint rboID = 0;
     GLuint texID = 0;
+    GLint int_fmt = 0;
     int w = 0;
     int h = 0;
 
@@ -16,6 +17,7 @@ public:
     ~RTT();
 
     void destroy();
+    void resize(int width, int height);
     bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8);
     bool recreate() { return create(w, h); }
     void clear(glm::vec4 color = glm::vec4(0));