WebGLRenderer: Allow for binding and rendering into a 2d render target mipmap (#29844)

* Allow for binding and rendering into a render target mipmap

* Fix MSRT mipmap rendering

src/renderers/WebGLRenderer.js

@@ -1272,7 +1272,7 @@ class WebGLRenderer {
 
 			//
 
-			if ( _currentRenderTarget !== null ) {
+			if ( _currentRenderTarget !== null && _currentActiveMipmapLevel === 0 ) {
 
 				// resolve multisample renderbuffers to a single-sample texture if necessary
 
@@ -2267,6 +2267,7 @@ class WebGLRenderer {
 
 		};
 
+		const _scratchFrameBuffer = _gl.createFramebuffer();
 		this.setRenderTarget = function ( renderTarget, activeCubeFace = 0, activeMipmapLevel = 0 ) {
 
 			_currentRenderTarget = renderTarget;
@@ -2375,6 +2376,14 @@ class WebGLRenderer {
 
 			}
 
+			// Use a scratch frame buffer if rendering to a mip level to avoid depth buffers
+			// being bound that are different sizes.
+			if ( activeMipmapLevel !== 0 ) {
+
+				framebuffer = _scratchFrameBuffer;
+
+			}
+
 			const framebufferBound = state.bindFramebuffer( _gl.FRAMEBUFFER, framebuffer );
 
 			if ( framebufferBound && useDefaultFramebuffer ) {
@@ -2395,8 +2404,15 @@ class WebGLRenderer {
 			} else if ( isRenderTarget3D ) {
 
 				const textureProperties = properties.get( renderTarget.texture );
-				const layer = activeCubeFace || 0;
-				_gl.framebufferTextureLayer( _gl.FRAMEBUFFER, _gl.COLOR_ATTACHMENT0, textureProperties.__webglTexture, activeMipmapLevel || 0, layer );
+				const layer = activeCubeFace;
+				_gl.framebufferTextureLayer( _gl.FRAMEBUFFER, _gl.COLOR_ATTACHMENT0, textureProperties.__webglTexture, activeMipmapLevel, layer );
+
+			} else if ( renderTarget !== null && activeMipmapLevel !== 0 ) {
+
+				// Only bind the frame buffer if we are using a scratch frame buffer to render to a mipmap.
+				// If we rebind the texture when using a multi sample buffer then an error about inconsistent samples will be thrown.
+				const textureProperties = properties.get( renderTarget.texture );
+				_gl.framebufferTexture2D( _gl.FRAMEBUFFER, _gl.COLOR_ATTACHMENT0, _gl.TEXTURE_2D, textureProperties.__webglTexture, activeMipmapLevel );
 
 			}