Shader uniform naming should be handled better

[Bil Bas]

At the moment, a uniform can be accessed as "in_VarName" or :var_name (used, in Shader#initialize, for example). Alternatively one can call (dynamic) methods to set them, such as shader.var_name = 5

Definitely shouldn't be treating strings and symbols differently, because that will confuse people who think they are the same thing :D

Either should enforce a naming convention or allow anyone to use whatever, but the latter would mean you'd be doing shader.in_VarName if you wanted to be consistent with a common GLSL standard. Meh, all messy!