material: prevent different uniform types from being assigned to the same name

[jfcameron]

material holds uniform name-value pairs for all the different uniform value types. Currently nothing stops a user from assigning e.g: a float to "my_uniform" then later assigning a texture or vector to it. They will all be collected in the different maps, the value which will be available to the shader will be whichever one happens to line up with the type of the uniform declared in the shader. This isnt a big deal but could be a source of confusion

[jfcameron]

should be done after #36 since that ticket will result in refactoring part of material