colors: add alpha() function to modify constant colors easily

[naufraghi]

Like RED.alpha(0.5)

[naufraghi]

Just a suggestion, was handy in my exploration. Feel free to close the PR. Kudos for macroquad!

[not-fl3]

I like how RED.alpha(0.5) looks, and it is indeed shorter and cleaner than Color {a: 0.5, ..RED}
But I do not really like that other color components doesn't work like this and neither do vectors/any other structs we got.

To be honest I do not have a strong enough opinion on consistency vs convinience so I can't make a decision on merging it or not.

[naufraghi]

My ratio was:

1. a is a bit special as a color component, the color is the same, but with a special transparency feature. Changing rgb makes a new color,
2. Constant colors are a limited number, and usually used arbitrarily just to put something on the screen. In this setting it's handy for exploration to see below something, hence the alpha modifier.

But, I completely overlooked the Color {a: 0.5, ..RED} option, it works well enough without any unbalance in the API!