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

irr/include/EDriverFeatures.h

@@ -129,6 +129,9 @@ enum E_VIDEO_DRIVER_FEATURE
 	//! Support for clamping vertices beyond far-plane to depth instead of capping them.
 	EVDF_DEPTH_CLAMP,
 
+	//! Support for multisample textures.
+	EVDF_TEXTURE_MULTISAMPLE,
+
 	//! Only used for counting the elements of this enum
 	EVDF_COUNT
 };

irr/include/IRenderTarget.h

@@ -26,6 +26,7 @@ enum E_CUBE_SURFACE
 };
 
 //! Interface of a Render Target.
+/** This is a framebuffer object (FBO) in OpenGL. */
 class IRenderTarget : public virtual IReferenceCounted
 {
 public:

irr/include/ITexture.h

@@ -156,6 +156,9 @@ enum E_TEXTURE_TYPE
 	//! 2D texture.
 	ETT_2D,
 
+	//! 2D texture with multisampling.
+	ETT_2D_MS,
+
 	//! Cubemap texture.
 	ETT_CUBEMAP
 };

irr/include/IVideoDriver.h

@@ -273,6 +273,14 @@ public:
 	virtual ITexture *addRenderTargetTexture(const core::dimension2d<u32> &size,
 			const io::path &name = "rt", const ECOLOR_FORMAT format = ECF_UNKNOWN) = 0;
 
+	//! Adds a multisampled render target texture to the texture cache.
+	/** \param msaa The number of samples to use, values that make sense are > 1.
+	Only works if the driver supports the EVDF_TEXTURE_MULTISAMPLE feature,
+	check via queryFeature.
+	\see addRenderTargetTexture */
+	virtual ITexture *addRenderTargetTextureMs(const core::dimension2d<u32> &size, u8 msaa,
+			const io::path &name = "rt", const ECOLOR_FORMAT format = ECF_UNKNOWN) = 0;
+
 	//! Adds a new render target texture with 6 sides for a cubemap map to the texture cache.
 	/** \param sideLen Length of one cubemap side.
 	\param name A name for the texture. Later calls of getTexture() with this name will return this texture.
@@ -358,6 +366,10 @@ public:
 	//! Remove all render targets.
 	virtual void removeAllRenderTargets() = 0;
 
+	//! Blit contents of one render target to another one.
+	/** This is glBlitFramebuffer in OpenGL. */
+	virtual void blitRenderTarget(IRenderTarget *from, IRenderTarget *to) = 0;
+
 	//! Sets a boolean alpha channel on the texture based on a color key.
 	/** This makes the texture fully transparent at the texels where
 	this color key can be found when using for example draw2DImage