* explained a bit more the core_window_letterbox example

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

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

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

Fix build failure introduced in 2a913b6587

* Update year of copyright notices

* Fix mistake in comment

* Fix typo ("algorythms")

* Replace tabs with spaces

* Remove trailing whitespace and fix mistake in comment

* Fix ExportImageAsCode missing comment rectangle corner

* Replace tab with spaces

* Replace tabs with spaces

Changes:

 - Font structure has been redesigned, CharInfo structure contained character rectangle within font texture, it has not much sense, considering that it was an information relative to the font atlas generated and not the character itself, so character rectangles have been moved out from CharInfo to Font.
 - CharInfo included a data parameters to contain character pixel data (usually grayscale), generated on TTF font generation. It was inconsistent with other fonts types, so, now CharInfo includes directly an Image of the glyph.
 - REDESIGNED: GenImageFontAtlas(), additional recs parameter added, loaded and filled inside the function to export atlas characters rectangles, instead of silently modify the input CharInfo data.
 - REVIEWED: ImageTextEx(), funtion retrieved the font atlas image from the GPU, that was slow and problematic in several platforms. Now it uses directly the CharInfo image. Support for unicode codepoints has also been added.
 - REDESIGNED: ImageDraw(), now it includes an additional parameter, the color tint, not only it could be useful for several situations but also function signature is more consistent with similar functions.
 - ADDED: ImageFromImage() to generate a new image from a piece of another image.
 - REVIEWED: GetNextCodepoint(), renamed parameters to be more clear.

Also all examples and games that were affected by those changes have been reviewed.

Added security checks in case provided image to functions hasn't been properly loaded... just to avoid program crashes.

Users can define their custom memory management macros.

NOTE: Most external libraries support custom macros in the same way, raylib should redefine those macros to raylib ones, to unify custom memory loading. That redefinition is only implemented as example for stb_image.h in [textures] module.

REDESIGNED: rlLoadRenderTexture()
ADDED: rlRenderTextureAttach()
ADDED: rlRenderTextureComplete()

Probably uncomplete, not tested yet...

Useful for tiling and offset parameters definition.

Useful in case raylib compilation want to be automated and compilation config flags provided by command line.