Include alpha in visual shader light function inputs

Lack of access to the alpha accumulator makes some effects that are sensitive to lighting impossible with the visual shader graph. This is an easy fix thankfully as it simply just wasn't exposed.

scene/resources/visual_shader.cpp

@@ -3149,6 +3149,7 @@ const VisualShaderNodeInput::Port VisualShaderNodeInput::ports[] = {
 
 	// Node3D, Light
 	{ Shader::MODE_SPATIAL, VisualShader::TYPE_LIGHT, VisualShaderNode::PORT_TYPE_VECTOR_3D, "albedo", "ALBEDO" },
+	{ Shader::MODE_SPATIAL, VisualShader::TYPE_LIGHT, VisualShaderNode::PORT_TYPE_SCALAR, "alpha", "ALPHA" },
 	{ Shader::MODE_SPATIAL, VisualShader::TYPE_LIGHT, VisualShaderNode::PORT_TYPE_SCALAR, "attenuation", "ATTENUATION" },
 	{ Shader::MODE_SPATIAL, VisualShader::TYPE_LIGHT, VisualShaderNode::PORT_TYPE_VECTOR_3D, "backlight", "BACKLIGHT" },
 	{ Shader::MODE_SPATIAL, VisualShader::TYPE_LIGHT, VisualShaderNode::PORT_TYPE_SCALAR, "clip_space_far", "CLIP_SPACE_FAR" },