Add glm::abs() function for the glm::mat<> class

This is added as part of the GLM_EXT_matrix_common extension, as this
function is not provided by the core GLSL specification (version 4.20).

The implementation of this glm::abs(mat<>) function mirrors the
glm::abs(vec<>) implementation. It should be functionning the same in
every way as the vec implementation.

Bonus points : AFAICT this allows to vectorize operations on compilers
that support optimization of these patterns, just like the functor1<>
struct in _vectorize.hpp for vectors.

test/ext/ext_matrix_common.cpp

@@ -43,11 +43,41 @@ static int test_mix()
 	return Error;
 }
 
+static int test_abs()
+{
+	int Error = 0;
+
+	{
+		glm::mat4 A(
+			3.0f, 1.0f, 5.2f, 4.9f,
+			1.4f, 0.5f, 9.3f, 3.7f,
+			6.8f, 8.4f, 4.3f, 3.9f,
+			5.6f, 7.2f, 1.1f, 4.4f
+		);
+		glm::mat4 B(
+			 1.0,-1.0, 1.0, 1.0,
+			-1.0, 1.0, 1.0,-1.0,
+			 1.0,-1.0,-1.0,-1.0,
+			-1.0,-1.0, 1.0, 1.0
+		);
+		glm::mat4 C = glm::matrixCompMult(A, B); // Not * to avoid matrix product.
+		glm::mat4 D = glm::abs(C);
+		glm::bvec4 const row1 = glm::equal(D[0], A[0]);
+		glm::bvec4 const row2 = glm::equal(D[1], A[1]);
+		glm::bvec4 const row3 = glm::equal(D[2], A[2]);
+		glm::bvec4 const row4 = glm::equal(D[3], A[3]);
+		Error += glm::all(glm::bvec4{glm::all(row1), glm::all(row2), glm::all(row3), glm::all(row4)}) ? 0 : 1;
+
+		return Error;
+	}
+}
+
 int main()
 {
 	int Error = 0;
 
 	Error += test_mix();
+	Error += test_abs();
 
 	return Error;
 }