Use struct for mat4_t instead of array

gpu.h

@@ -6,12 +6,18 @@
 
 #define SWAPCHAIN_IMG_BUFFER_SIZE 4
 
-typedef float mat4_t[16];
-
 typedef struct {
 	float x, y, z;
 } vec3_t;
 
+typedef struct {
+	float x, y, z, w;
+} vec4_t;
+
+typedef struct {
+	vec4_t x, y, z, w;
+} mat4_t;
+
 typedef struct {
 	vec3_t position;
 	vec3_t colour;