shadeHalveAndTrade(reducingShaderFunc) {
        this.shadeAndTrade(
            reducingShaderFunc,
            WglTexturePool.take(Math.max(0, this.currentTexture.sizePower() - 1), this.currentTexture.pixelType))
    }
Renders to a texture that's a half the size of the current texture, then uses the rendered texture as the new
current texture.

@param {!function(!WglTexture) : !WglConfiguredShader} reducingShaderFunc