Improvements to handling of disconnected split keyboards. (#14033)

* Use memcmp and memcpy to compare and copy slave matrix.

...and memset to initialize `matrix` and `raw_matrix`.

Increased my scan rate (while connected) by ~100 (on Ergodox Infinity).
Effect on AVR is unknown.

Co-authored-by: Stefan Kerkmann <karlk90@pm.me>

* Fix `matrix_post_scan` signalling change on every scan while disconnected.

* Undo removal of initialization of `slave_matrix`.

This has the effect of increasing my Ergodox Infinity firmware size by 8
bytes instead of decreasing by 8 bytes, and lowers the scan rate while
connected back to the initial value before these changes, but _might_
solve some issues on AVR.

Co-authored-by: Stefan Kerkmann <karlk90@pm.me>

quantum/matrix.c

@@ -288,10 +288,8 @@ void matrix_init(void) {
     matrix_init_pins();
 
     // initialize matrix state: all keys off
-    for (uint8_t i = 0; i < MATRIX_ROWS; i++) {
-        raw_matrix[i] = 0;
-        matrix[i]     = 0;
-    }
+    memset(matrix, 0, sizeof(matrix));
+    memset(raw_matrix, 0, sizeof(raw_matrix));
 
     debounce_init(ROWS_PER_HAND);
 
@@ -312,24 +310,22 @@ __attribute__((weak)) bool transport_master_if_connected(matrix_row_t master_mat
 bool matrix_post_scan(void) {
     bool changed = false;
     if (is_keyboard_master()) {
+        static bool  last_connected              = false;
         matrix_row_t slave_matrix[ROWS_PER_HAND] = {0};
         if (transport_master_if_connected(matrix + thisHand, slave_matrix)) {
-            for (int i = 0; i < ROWS_PER_HAND; ++i) {
-                if (matrix[thatHand + i] != slave_matrix[i]) {
-                    matrix[thatHand + i] = slave_matrix[i];
-                    changed              = true;
-                }
-            }
-        } else {
-            // reset other half if disconnected
-            for (int i = 0; i < ROWS_PER_HAND; ++i) {
-                matrix[thatHand + i] = 0;
-                slave_matrix[i]      = 0;
-            }
+            changed = memcmp(matrix + thatHand, slave_matrix, sizeof(slave_matrix)) != 0;
 
+            last_connected = true;
+        } else if (last_connected) {
+            // reset other half when disconnected
+            memset(slave_matrix, 0, sizeof(slave_matrix));
             changed = true;
+
+            last_connected = false;
         }
 
+        if (changed) memcpy(matrix + thatHand, slave_matrix, sizeof(slave_matrix));
+
         matrix_scan_quantum();
     } else {
         transport_slave(matrix + thatHand, matrix + thisHand);