texture.mipmaps value needs to be updated, so, texture must be passed by
reference instead of by value

SetTextureFilter()
SetTextureWrap()

Support grayscale (8 bit) textures for fonts
Load unordered chars data above char 126

Removed support for some unusual image formats

raylib uses spaces between '+' and '-' signs but not between '*' and '/'
signs, it's a chosen convention

Moved all external libraries used by raylib to external folder inside
raylib src. Makefile has already been update and also the different
includes in raylib modules.

This is a first step toward a bigger project. Some modules could be
ported to header-only to be used as standalone.

Support render texture size different than screen size

Now it's possible to render to texture, old postprocessing system will
be removed on next raylib version.

Specially useful on default font scaling

Corrected some code details

Updated functions:
Image GetTextureData(Texture2D texture);
void *rlglReadTexturePixels(Texture2D texture);

LoadTextureEx() - Simplified parameters
UpdateTexture() - Added, allows updating GPU texture data

Added (or completed functions):

Image ImageText(const char *text, int fontSize, Color color);
Image ImageTextEx(SpriteFont font, const char *text, int fontSize, int
spacing, Color tint);
void ImageFlipVertical(Image *image);
void ImageFlipHorizontal(Image *image);
void ImageColorTint(Image *image, Color color);
void ImageColorInvert(Image *image);
void ImageColorGrayscale(Image *image);
void ImageColorContrast(Image *image, float contrast);
void ImageColorBrightness(Image *image, int brightness);

Renamed functions (for coherence with new ones):
- ImageConvertToPOT() -> ImageToPOT()
- ImageConvertFormat() -> ImageFormat()

New functions added (IN PROGRESS):
- ImageCopy()
- ImageCrop()
- ImageResize() (Uses stb_image_resize.h)
- ImageDraw()
- ImageDrawText()
- ImageDrawTextEx()
- ImageFlipVertical()
- ImageFlipHorizontal()
- ImageColorInvert()
- ImageColorGrayscale()
- ImageColorContrast()
- ImageColorBrightness()

[rlgl] Extensions strings loading was redone to avoid a Segmentation
Fault on Raspberry Pi

Added:
- LoadImageRaw()
- ImageCopy()
Renamed:
- GetPixelData() -> GetImageData()

Corrected issues with textures loading

Complete review of matrix rotation math
Check compressed textures support
WIP: LoadImageFromData()