Add Frame.createHalfFloat

[finscn]

Detail at #32

[finscn]

I know in current pixi-gl-core , you check the type of GLTexture in Texture.prototype.uploadData
You use if(data instanceof Float32Array)... .

But for half-float , it's not a good idea.

Because half-float supports null-data , and sometimes we have to use null.

So, I do all things about create half-float in Frame.createHalfFloat.

[finscn]

Another thing, in safair (iOS & mac) , the half-float GLTexture doesn't support uploadData.
User can't change its content via upload arraybuffer , it only could be changed via shader.

And in safair (iOS & mac) , float32 GLTexture is NOT renderable , it only could be changed via upload arraybuffer.

It's so strange..... I don't know what Apple's developers are thinking...

[finscn]

A better way is refactor Texture.prototype.uploadData or refactor Texture's constructor.
But in current version , I think , do many change is not a good choice . So I choose add new function , don't touch existing functions.