Add alpha support for textures

[matt-pan]

Hi,

One small suggestion from my side for this very helping tutorial would be to change the color vec3 to a vec4 so that textures can have an alpha value applied too. The Color class has an alpha member (and getter/setter) already but it is not used by the Renderer at all. I tried to change this myself but apparently I wasn't able to do it right.

[Heiko Brumme]

Thanks for the Suggestion. I will look into this.

[matt-pan]

Thanks a lot