Fixed HUD components appearing in water reflection

Clear the region on which the world is rendered (vertically flipped)
to create the reflection texture - otherwise regions not painted over
still contain data from the previous frame, including some HUD elements
in the bottom-left corner of the screen.

source/src/rendergl.cpp

@@ -668,6 +668,8 @@ void drawreflection(float hf, int w, int h, float changelod, bool refract)
         glScissor(sx, sy, sw, sh);
         glEnable(GL_SCISSOR_TEST);
     }
+    
+    glClear(GL_COLOR_BUFFER_BIT); // Fix HUD components appearing in reflection
 
     resetcubes();