[lib][gfx][mandelbrot] make sure alpha channel is set
[lib][gfx][mandelbrot] make sure alpha channel is set

stm32f7 discovery board apparently requires the alpha channel to be set.

hollanderic/lkstuff,hollanderic/lkstuff,hollanderic/lkstuff,travisg/lk,littlekernel/lk,travisg/lk,littlekernel/lk,travisg/lk,hollanderic/lkstuff,littlekernel/lk,travisg/lk,littlekernel/lk