int to std::size_t to match ofMatrix4x4

[dimitre]

and it fixes some more strict compiler warnings "Implicit conversion loses integer precision:"
And some space to tabs fix.

Let me know if it is best to make different PRs for readability
only changed code was this lines

	float& operator[]( std::size_t n ){
	float operator[]( std::size_t n ) const {

instead of

	float& operator[]( int n ){
	float operator[]( int n ) const {

[dimitre]

@ofTheo I'll be merging this one as it was reviewed by @artificiel as good to merge here: #7741