TextInput fix on x86 Windows

On 32-bit Windows a TextInput event receives multiple strange characters (after the first valid char), characters that, by default, cannot be rendered by a SpriteFont. These characters are strange, and they are different every time I start the game. This patch ignores these strange characters and just sends off the valid one. I remember reading somewhere on the SDL2 website that "the first character will always be what the client presses" - not sure if this is related to that. Ive tried searched thoroughly for this page but I am unable to find.

Ive actually had this fix for a while now for my own game, I thought it would be smart to get the fix here now ASAP so its ready for MonoGame repo merge

I will be attempting playing song's again soon as I see MonoGameSDL is making its move to the official repository, which I am very excited for :dancer: