Testing issue qmk#1191 (n-column split i2c slave)

Based on initial OrthoDox (serial) config by @reddragond and others,
this attempts to add TWI (I2C) support.
Relevant: <qmk#1191>

- per @ahtn recommendation, using memcpy for moving slave matrix
  into slave sending buffer
- slave buffer has been enlarged using sizeof(matrix_row_t)
- note: i2c.h now includes matrix.h
- note: matrix.c includes <string.h>

keyboards/orthodox/i2c.h

@@ -2,6 +2,7 @@
 #define I2C_H
 
 #include <stdint.h>
+#include "matrix.h"
 
 #ifndef F_CPU
 #define F_CPU 16000000UL
@@ -13,7 +14,7 @@
 #define I2C_ACK 1
 #define I2C_NACK 0
 
-#define SLAVE_BUFFER_SIZE 0x10
+#define SLAVE_BUFFER_SIZE (MATRIX_ROWS/2*sizeof(matrix_row_t))
 
 // i2c SCL clock frequency
 #define SCL_CLOCK  400000L

keyboards/orthodox/matrix.c

@@ -20,6 +20,10 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 #include <stdint.h>
 #include <stdbool.h>
+#ifdef USE_I2C
+// provides memcpy for copying TWI slave buffer
+#include <string.h>
+#endif
 #include <avr/io.h>
 #include <avr/wdt.h>
 #include <avr/interrupt.h>
@@ -167,11 +171,13 @@ int i2c_transaction(void) {
     if (err) goto i2c_error;
 
     if (!err) {
-        int i;
-        for (i = 0; i < ROWS_PER_HAND-1; ++i) {
-            matrix[slaveOffset+i] = i2c_master_read(I2C_ACK);
+	// read from TWI byte-by-byte into matrix_row_t memory space
+        size_t i;
+        for (i = 0; i < SLAVE_BUFFER_SIZE-1; ++i) {
+            *((uint8_t*)&matrix[slaveOffset]+i) = i2c_master_read(I2C_ACK);
         }
-        matrix[slaveOffset+i] = i2c_master_read(I2C_NACK);
+	// last byte to be read / end of chunk
+        *((uint8_t*)&matrix[slaveOffset]+i) = i2c_master_read(I2C_NACK);
         i2c_master_stop();
     } else {
 i2c_error: // the cable is disconnceted, or something else went wrong
@@ -236,10 +242,8 @@ void matrix_slave_scan(void) {
     int offset = (isLeftHand) ? 0 : (MATRIX_ROWS / 2);
 
 #ifdef USE_I2C
-    for (int i = 0; i < ROWS_PER_HAND; ++i) {
-        /* i2c_slave_buffer[i] = matrix[offset+i]; */
-        i2c_slave_buffer[i] = matrix[offset+i];
-    }
+    // SLAVE_BUFFER_SIZE is from i2c.h
+    memcpy((void*)i2c_slave_buffer, (const void*)&matrix[offset], SLAVE_BUFFER_SIZE);
 #else // USE_SERIAL
     for (int i = 0; i < ROWS_PER_HAND; ++i) {
         serial_slave_buffer[i] = matrix[offset+i];