Get the OES_texture_float_linear extension.

There's a bug in most WebGL implementations that the OES_texture_float
extension is not supposed to allow the various LINEAR filtering modes.
To filter a floating point texture with linear filtering requires
enabling OES_texture_float_linear.

This CL does that. The code has not been updated to actually
check the extension exists and doing or disallowing the appropriate
features.

Enabling it will mean on desktop things just work as they did before.

On mobile/tablets it's far more likley OES_texture_float_linear
does not exist

src/renderers/WebGLRenderer.js

@@ -191,6 +191,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 	var _gl;
 
 	var _glExtensionTextureFloat;
+	var _glExtensionTextureFloatLinear;
 	var _glExtensionStandardDerivatives;
 	var _glExtensionTextureFilterAnisotropic;
 	var _glExtensionCompressedTextureS3TC;
@@ -7277,6 +7278,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 		}
 
 		_glExtensionTextureFloat = _gl.getExtension( 'OES_texture_float' );
+		_glExtensionTextureFloatLinear = _gl.getExtension( 'OES_texture_float_linear' );
 		_glExtensionStandardDerivatives = _gl.getExtension( 'OES_standard_derivatives' );
 
 		_glExtensionTextureFilterAnisotropic = _gl.getExtension( 'EXT_texture_filter_anisotropic' ) ||