Extend exported material parameters

Add next parameters:
 * ambient intensity  - amount of ambient light the mesh receives;
 * diffuse intensity  - amount of diffuse light the mesh reflects;
 * diffuse color      - diffuse color in normalized rgb;
 * specular intensity - the intensity of mesh specular reflection;
 * specular hardness  - the hardness of mesh specular reflection;
 * specular color     - specular color in normalized rgb.
Extend G-Buffer with 4th texture (specular color)

Move constructor to .cpp file.
Implement Material class
Add uniform block support into Shader class

Update default shaders code with Material uniform block.
Improve performance

Reduce the number of glUseProgram calls for a single program.
Remove depth texture from FrameBuffer.
Disable depth test for frame rendering, enable blending instead.
Read all material parameters from entity file
Update example entity