add explicit quat to mat3/mat4 type conversion

[plasmacel]

It would be useful if glm::mat3 / glm::mat4 types had explicit constructors accepting glm::quat
to make full support for standard conversion between quat / mat types and vice versa. glm::quat already implements this behaviour for mat types. it should be supported vice versa.

[Groovounet]

GTC_quaternion supports quat_cast, mat3_cast and mat4_cast already.

Adding these constructors would add a dependency of core code (what GLSL supports) to extensions (what GLSL doesn't support). This is going against the spirit of the library.

Thanks,
Christophe