Changed Mesa backend to output GLSL ES 3.00 in order to support shadow mapping
Disabled shadow mapping for Mesa drivers fixes #103
Patched GLEW to get the extensions with OpenGL >= 3.2 for the case that glGetString(GL_EXTENSIONS) fails
Removed glewExperimental = GL_TRUE hack for SDL 2
Added r_forceShadowMapsOnAlphaTestedSurfaces 1
Updated embedded shaders