Simplify matrix declarations

[dhritzkiv]

if diagonal elements are all the same, and the rest are zeros

e.g.

mat3 fullForm = mat3(
	1.0, 0.0, 0.0,
	0.0, 1.0, 0.0,
	0.0, 0.0, 1.0
);

mat3 shortForm = mat3(1.0);

fullForm == shortForm;

[coveralls]

Coverage decreased (-0.7%) to 99.275% when pulling 07f3ce1 on minify-mats into a284825 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 05291cf on minify-mats into a284825 on master.