Refine default Camera3D parameters

[Bigfoot71]

I have adjusted the default parameters of Camera3D for a more consistent configuration:

• The default target is now the standard forward {0,0,-1} instead of {0,0,0}.
• The fovy is now set to 45° instead of 0, matching the value used in most raylib examples.

[RobLoach]

Seems appropriate to me. Thanks.