GLFW mappings.

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);

This reverts commit 949e93f9fe.

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()

- Renamed function rlEnableFBO() -> rlEnablePostproFBO()
- Defined struct FBO
- Moved FBO creation to function: rlglLoadFBO()
- Reviewed rlglReadTexturePixels(), trying to support OpenGL ES -IN
PROGRESS-

Time measure on Windows platform done with:
QueryPerformanceCounter()
Testing required!

Corrected bug on defaultFont.size
Renamed funcs:
ImageConvertFormat() -> ImageFormat()
ImageConvertToPOT() -> ImageToPOT()

...shapes based on LINES, TRIANGLES and QUADS. Now the calling order of
the drawing functions is respected!

strdup() is not a C99 function (it's POSIX), not available in emscripten

On OpenGL ES it was set to use GL_CLAMP_TO_EDGE wrap mode for textures.
On LoadOBJ() texture coordinates were wrongly Y-flipped

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

Splitted into:
UpdateCamera()             // Update only camera
UpdateCameraPlayer()   // Update camera and player position

Immediate Mode GUI (IMGUI)

- Trying to implement rlglUnproject() - NOT WORKING
- Tested GLEW alternatives: glad and glLoadGen - NOT WORKING

Useful to enable additive blend mode for particles

Requires raymath as standalone and GLEW

...shader loading security check