Color4: Fix << operator for Color4<uint8>
Color4: Introduce << operator for Color4f
SDL_helpers: Add nullptr checks before freeing

* Doing nullptr checks before freeing helps eliminate edge cases
TrueTypeFont: Add additional validity checks

* Fix a few segmentation faults that occur otherwise, such as if you
were to try setting the text before loading the font
TrueTypeFont: Remove unused draw method overload
Image: Introduce several new methods

* Add pixel_format argument onto load, load_bmp methods; BitmapFont
class needs to be in a particular pixel format...

* Introduce set_alpha method
* Introduce getter & setter methods position & set_position with its
instance variable position_
BitmapFont: Refactor way we access video memory

* Use static access texture instead of a streaming access texture
app: Move sprites, fonts, UI out of window loop
Revert commit 688b5f1

* Relocating to SDL_App didn't pan out (crashes in TTcards);
SDL_App destructs long before our TrueTypeFont class does, which does
not allow us to free our font resource afterwards!
Texture: Add temporary workaround patch for crash

See Issue #8