Implement support for FSAA in combination with post-processing (#15392)

- Actually it's MSAA I think, or perhaps the terms are equivalent
- I've made it fit into the existing Irrlicht architecture, but that has resulted in code duplication compared to my original "hacky" approach
- OpenGL 3.2+ and OpenGL ES 3.1+ are supported
- EDT_OPENGL3 is not required, EDT_OPENGL works too
- Helpful tutorial: https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing, section "Off-screen MSAA"
- This may be rough around the edges, but in general it works

builtin/settingtypes.txt

@@ -410,10 +410,12 @@ anisotropic_filter (Anisotropic filtering) bool false
 #    * None - No antialiasing (default)
 #
 #    * FSAA - Hardware-provided full-screen antialiasing
-#             (incompatible with Post Processing and Undersampling)
 #             A.K.A multi-sample antialiasing (MSAA)
 #             Smoothens out block edges but does not affect the insides of textures.
-#             A restart is required to change this option.
+#
+#             If Post Processing is disabled, changing FSAA requires a restart.
+#             Also, if Post Processing is disabled, FSAA will not work together with
+#             undersampling or a non-default "3d_mode" setting.
 #
 #    * FXAA - Fast approximate antialiasing
 #             Applies a post-processing filter to detect and smoothen high-contrast edges.