Add setDisplaySize method in BitmapText

[rexrainbow]

Please do not update the README or Change Log, we will do this when we merge your PR.

This PR (delete as applicable)

• Adds a new feature

Describe the changes below:

Since BitmapText does not have original size, I set scaleX, scaleY to 1 to get original size,
then calculate new scaleX, scaleY by displayWidth, displayHeight.

[photonstorm]

Does it fix something to add this? Or is it just a feature?

[rexrainbow]

Since displayWidth, displayHeight properties does not have setter, I thought it is a new feature.

Because that BitmapText has scaleX, scaleY properties, it can have setter of displayWidth, displayHeight propertie.
But BitmapText does not store original size (size when scale is 1) directly, I have to get this original size first. This solution is not straightforward I agree.

[photonstorm]

I think I'd rather just store the original size somewhere and use that. Although I guess it will need resetting every time you change the font size, for example.

[rexrainbow]

I think I'd rather just store the original size somewhere and use that. Although I guess it will need resetting every time you change the font size, for example.

It is a better solution.