Replaced by fabs() that seem to work ok

On OpenGL 1.1 only is supported for 32bit RGBA textures

- Renamed Begin3dMode() --> BeginMode3D()
- Renamed Begin2dMode() --> BeginMode2D()
- Renamed End3dMode() --> EndMode3D()
- Renamed End2dMode() --> EndMode2D()

That way, a user needs only to touch a single file to configure what
features raylib is built with.
Include guards are left out intentionally, because config.h should only
be included in source files, not headers.

Later on, config.h can also define the raylib version (#461).

Added struct Vector4 for convenience

Allows loading of shader from text code string directly

When changing image format, mipmaps are lost and regenerated from
scratch

Review rlLoadTexture() function to make it simpler, now OpenGL texture
glInternalFormat, glFormat and glType are retrieved with new function
GetGlFormats()

- Support OpenGL 1.1 if selected macro
- Corrected MSVC compiler issues with (void *) data
- Removed raylib.dll project
- Created DEBUG_DLL and RELEASE_DLL configurations

- Added support for mipmap levels loading
- Removed internal function LoadTextureCompressed(), not required any
more...

Matrix stack system should be reviewed but, in the meantime,
currentMatrix should be reseted in order of 3d to work

Note that this solution is a temporal patch, not properly tested and
prone to fail, specially if matrix are pushed into the stack...

- Renamed enum TextureFormat to PixelFormat for consistency
- Added support for pixel format UNCOMPRESSED_R32
- Using GetPixelDataSize() where required

If shader file could not be found on loading it crashed, instead added
fallback to default shader!

By default, we look for texture1 for LOC_MAP_SPECULAR and texture2 for
LOC_MAP_NORMAL

Support Universal Windows Platform (UWP):
- Windows 10 App
- Windows Phone
- Xbox One

USE_DEPTH_RENDERBUFFER on OpenGL 2.1

On RPI, after latest drivers update, models were not rendered properly
cause of that...

- Review RPI compilation (core_drop_files not supported)
- Review ImageFormat(), some issues
- GetTextureData() reviewed for RPI

Improve second option to retrieve texture data from fbo...

Still requires some testing...

...while reviewing GetTextureData() functionality on OpenGL ES 2.0

To be more generic and configurable

It should be reviewed more carefully...

Requires some work, distortion shader could be move out of raylib to
example code...

to emulate OpenGL funcionality