GetPixelAtPoint() function for texture class

[matt-comm-link]

I've been working on a function to get the colour of a pixel (as a Vec4) at a specified location for heightmap terrain in a project I'm working on.

I'd like to propose adding a function like that into the texture class proper. I'm still working on my implementation, and I'm not familiar enough with 4bpp pngs to have implemented anything for that just yet, so I won't submit a PR at the moment, but am thinking of doing so when I've battle tested things a little more.

[Wellinator]

That'd be very helpful. I've needed it before.