Use Active/BindTexture wrapper methods
internal/Graphics: _GL->ACTIVE_TEXTURE starts GL_TEXTURE0
internal/Graphics: Remove various implementation data

Data like MAX_TEXTURE_UNITS, etc. This data is accessed through
function calls, which is unnecessary -- we can set up an extern
variable in the internal/Graphics header and define the variable
inside the source file, where we can modify it when initializing
the graphics module.