Corrected bug when loading a RenderTexture texture passing NULL data.

* explained a bit more the core_window_letterbox example

* fixed a few 'ups' moments that could lead to mild head pain and time loss

ADDED: CodepointToUtf8()

RENAMED: TextCountCodepoints() -> GetCodepointsCount()

* Initial commit of addition for GLTF materials.. should support loading more than just albedo map.

* Clean up

* fixed seg faults and leaks

* temp don't overwrite defuse colour when rendering

* undid something dumb!

* correctly mixed diffuse map color when rendering to preserve not overwrite it

- Improved LoadBMFont()

Now it accepts a ';' separated list of extensions, useful to check multiple extensions

Get the unicode equivalent characters (as int array) from a UTF-8 text array... maybe this function is renamed to be clearer to users...

When adding alpha mask to GRAYSCALE image, if using ImageFormat(), it could change pixel values, now we avoid that, mask is just added as an extra channel

The result of `success` is actually never used. Either we should check
for it and return or remove it. I assume just checking the last one is
okay.

* Const corectness for LoadShaderCode

* Added const for standalone variant as well.

Still looking for a better way to integrate it into raylib API, maybe add a LoadImageAnim()?

Note that latest emsdk changed paths