Update CHANGELOG.md

Author: photonstorm

CHANGELOG.md

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 ## Version 3.19.0 - Naofumi - in development
 
 
+### New Features
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+* `Shader.setRenderToTexture` is a new method that will redirect the Shader to render to its own framebuffer / WebGLTexture instead of to the display list. This allows you to use the output of the shader as an input for another shader, by mapping a sampler2D uniform to it. It also allows you to save the Shader to the Texture Manager, allowing you to use it as a texture for any other texture based Game Object such as a Sprite.
+* `Shader.renderToTexture` is a new property flag that is set if you set the Shader to render to a texture.
+* `Shader.framebuffer` is a new property that contains a WebGLFramebuffer reference which is set if you set the Shader to render to a texture.
+* `Shader.glTexture` is a new property that contains a WebGLTexture reference which is set if you set the Shader to render to a texture.
+* `Shader.texture` is a new property that contains a Phaser Texture reference which is set if you set the Shader to save to the Texture Manager.
+* `TextureManager.addGLTexture` is a new method that allows you to add a WebGLTexture directly into the Texture Manager, saved under the given key.
+
 ### Bug Fixes
 
 * The Scale Manager would throw the error 'TypeError: this.removeFullscreenTarget is not a function' when entering full-screen mode. It would still enter fullscreen, but the error would appear in the console. Fix #4605 (thanks @darklightcode)
 
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 ## Version 3.18.1 - Raphtalia - 20th June 2019
 
 ### Bug Fixes