fix MG::Color#inspect in order to show valid values
fix MG::Color#inspect in order to show valid values

`Color#alpha` indicates 0.0 ~ 1.0 range value.
However `Color#inspect` show the value outside 0.0 ~ 1.0 range.

* Reproduction
```
@color = MG::Color.new
p @color.alpha
p @color
```

* Result
```
1.0
#<Color Red:0 Green:0 Blue:0 Alpha:255>
```

HipByte/motion-game,HipByte/motion-game,casper91/motion-game,casper91/motion-game,casper91/motion-game,casper91/motion-game,HipByte/motion-game,HipByte/motion-game