fix for intel graphics media accelerator card (GL 2.1)

hello,

could you guys look into this?
i had to tweak the extension loader to get bgfx to work in a legacy video card (2.1). 
all examples are working now, but one. #14 is asserting because i dont think half_float is supported on 2.1

other than that, top notch :)
lib looks promising and very clean, congrats :D
keep up the good work