math: use extern struct for guaranteed memory layout

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/math/mat.zig

@@ -10,7 +10,7 @@ pub fn Mat(
     comptime n_rows: usize,
     comptime Vector: type,
 ) type {
-    return struct {
+    return extern struct {
         v: [cols]Vec,
 
         /// The number of columns, e.g. Mat3x4.cols == 3

src/math/quat.zig

@@ -6,7 +6,7 @@ const math = mach.math;
 const vec = @import("vec.zig");
 
 pub fn Quat(comptime Scalar: type) type {
-    return struct {
+    return extern struct {
         v: vec.Vec(4, Scalar),
 
         /// The scalar type of this matrix, e.g. Mat3x3.T == f32

src/math/vec.zig

@@ -5,7 +5,7 @@ const testing = mach.testing;
 const math = mach.math;
 
 pub fn Vec(comptime n_value: usize, comptime Scalar: type) type {
-    return struct {
+    return extern struct {
         v: Vector,
 
         /// The vector dimension size, e.g. Vec3.n == 3