Fix T73169: Side by Side Stereo Rendering Glitches

Side by side and top down views were rendered using an unset matrix.
This fix will reset the matrix just before copying the views to the
screen.

Reviewed By: Clément Foucault, Dalai Felinto

Differential Revision: https://developer.blender.org/D7777

source/blender/windowmanager/intern/wm_draw.c

@@ -848,6 +848,7 @@ static void wm_draw_window(bContext *C, wmWindow *win)
         glActiveTexture(GL_TEXTURE0);
         glBindTexture(GL_TEXTURE_2D, GPU_texture_opengl_bindcode(texture));
 
+        wmWindowViewport(win);
         if (win->stereo3d_format->display_mode == S3D_DISPLAY_SIDEBYSIDE) {
           wm_stereo3d_draw_sidebyside(win, view);
         }