Fix glslscale bug when binding a framebuffer before gl_swap_buffers. (#…

…425)

docs/changelog.txt

@@ -7,6 +7,9 @@ USERS
 
 + All "gamecontroller[...]" config.txt options have been renamed
   to "gamepad[...]". The old names are still supported.
++ Fixed blank screen bugs that occur with the glslscale renderer
+  in conjunction with Intel HD Graphics 2500 graphics drivers
+  and possibly some other old drivers.
 + The softscale renderer now respects disable_screensaver.
 + HTML5: fixed the poor performance of FREADn and other features
   that rely on calculating the length of an open file.

src/render_glsl.c

@@ -1491,30 +1491,29 @@ static void glsl_sync_screen(struct graphics_data *graphics)
     gl_check_error();
   }
 
+  gl_swap_buffers(graphics);
+  render_data->dirty_palette = true;
+  render_data->dirty_indices = true;
+
   if(glsl.has_fbo)
   {
+    /* Switch back to the texture framebuffer. This needs to be done after
+     * gl_swap_buffers or the screen won't draw properly for some drivers.
+     * The SDL documentation cites macOS, but MegaZeux bugs were encountered
+     * using the Intel HD Graphics 2500 driver in Windows too.
+     */
     glsl.glBindFramebuffer(GL_FRAMEBUFFER, render_data->fbos[FBO_SCREEN_TEX]);
     gl_check_error();
-
-    // Clear the framebuffer so elements of this frame don't bleed through.
-    glsl.glClear(GL_COLOR_BUFFER_BIT);
-    gl_check_error();
   }
 
-  gl_swap_buffers(graphics);
-  render_data->dirty_palette = true;
-  render_data->dirty_indices = true;
-
   /**
+   * Clear the framebuffer so elements of this frame don't bleed through.
    * When FBOs are not available, the next window frame needs to be cleared
    * since the screen will be temporarily drawn there before scaling. This call
-   * was previously disabled for Emscripten (which should always have FBOs).
+   * was previously disabled for Emscripten.
    */
-  if(!glsl.has_fbo)
-  {
-    glsl.glClear(GL_COLOR_BUFFER_BIT);
-    gl_check_error();
-  }
+  glsl.glClear(GL_COLOR_BUFFER_BIT);
+  gl_check_error();
 }
 
 static boolean glsl_switch_shader(struct graphics_data *graphics,