WebGLTexture: Fix updateMultisampleRenderTarget(). (#21570)

* WebGLTexture: Fix updateMultisampleRenderTarget().

* WebGLState: Clean up.

src/renderers/webgl/WebGLState.js

@@ -447,6 +447,24 @@ function WebGLState( gl, extensions, capabilities ) {
 
 			currentBoundFramebuffers[ target ] = framebuffer;
 
+			if ( isWebGL2 ) {
+
+				// gl.DRAW_FRAMEBUFFER is equivalent to gl.FRAMEBUFFER
+
+				if ( target === gl.DRAW_FRAMEBUFFER ) {
+
+					currentBoundFramebuffers[ gl.FRAMEBUFFER ] = framebuffer;
+
+				}
+
+				if ( target === gl.FRAMEBUFFER ) {
+
+					currentBoundFramebuffers[ gl.DRAW_FRAMEBUFFER ] = framebuffer;
+
+				}
+
+			}
+
 		}
 
 	}

src/renderers/webgl/WebGLTextures.js

@@ -1201,11 +1201,6 @@ function WebGLTextures( _gl, extensions, state, properties, capabilities, utils,
 
 			if ( isWebGL2 ) {
 
-				const renderTargetProperties = properties.get( renderTarget );
-
-				state.bindFramebuffer( _gl.READ_FRAMEBUFFER, renderTargetProperties.__webglMultisampledFramebuffer );
-				state.bindFramebuffer( _gl.DRAW_FRAMEBUFFER, renderTargetProperties.__webglFramebuffer );
-
 				const width = renderTarget.width;
 				const height = renderTarget.height;
 				let mask = _gl.COLOR_BUFFER_BIT;
@@ -1213,9 +1208,15 @@ function WebGLTextures( _gl, extensions, state, properties, capabilities, utils,
 				if ( renderTarget.depthBuffer ) mask |= _gl.DEPTH_BUFFER_BIT;
 				if ( renderTarget.stencilBuffer ) mask |= _gl.STENCIL_BUFFER_BIT;
 
+				const renderTargetProperties = properties.get( renderTarget );
+
+				state.bindFramebuffer( _gl.READ_FRAMEBUFFER, renderTargetProperties.__webglMultisampledFramebuffer );
+				state.bindFramebuffer( _gl.DRAW_FRAMEBUFFER, renderTargetProperties.__webglFramebuffer );
+
 				_gl.blitFramebuffer( 0, 0, width, height, 0, 0, width, height, mask, _gl.NEAREST );
 
-				state.bindFramebuffer( _gl.FRAMEBUFFER, renderTargetProperties.__webglMultisampledFramebuffer ); // see #18905
+				state.bindFramebuffer( _gl.READ_FRAMEBUFFER, null );
+				state.bindFramebuffer( _gl.DRAW_FRAMEBUFFER, renderTargetProperties.__webglMultisampledFramebuffer );
 
 			} else {