prevent device block in SPI_MUTEX_LOCK() at least on ESP32

ESP32 uses a MUTEX to handle SPI transactions.
So calling beginTransaction twice in a row will block the device.
This happens in current code, if SRAM is used or a display (like 2,7" tricolor IL91874) which uses singleByteTxns = true
Both will call beginTransaction twice in a row and then the device will block for ever in
cores\esp32\esp32-hal-spi.h SPI_MUTEX_LOCK();

To prevent this, I made some changes in Adafruit_EPD.cpp
to track _isInTransaction properly, which was not used before.

See also adafruit/Adafruit-PN532#53 for a similar bug.

src/Adafruit_EPD.cpp

@@ -316,6 +316,7 @@ void Adafruit_EPD::writeSRAMFramebufferToEPD(uint16_t SRAM_buffer_addr,
 
   // use SRAM
   sram.csLow();
+  _isInTransaction = true;
   // send read command
   SPItransfer(MCPSRAM_READ);
   // send address
@@ -334,6 +335,7 @@ void Adafruit_EPD::writeSRAMFramebufferToEPD(uint16_t SRAM_buffer_addr,
   }
   csHigh();
   sram.csHigh();
+  _isInTransaction = false;
 }
 
 /**************************************************************************/
@@ -646,8 +648,10 @@ void Adafruit_EPD::csHigh() {
   digitalWrite(_cs_pin, HIGH);
 #endif
 
-  spi_dev->endTransaction();
-  _isInTransaction = false;
+  if (_isInTransaction) {
+    spi_dev->endTransaction();
+    _isInTransaction = false;
+  }
 }
 
 /**************************************************************************/
@@ -656,8 +660,11 @@ void Adafruit_EPD::csHigh() {
 */
 /**************************************************************************/
 void Adafruit_EPD::csLow() {
-  spi_dev->beginTransaction();
-  _isInTransaction = true;
+
+  if (!_isInTransaction) {
+    spi_dev->beginTransaction();
+    _isInTransaction = true;
+  }
 
 #ifdef BUSIO_USE_FAST_PINIO
   *csPort &= ~csPinMask;