Implement transaction based drawing

With using transactions to draw, we save huge amount of time in pin
changing and SPI setup. This new model, together with the updated
ILI9341 lib give a much better performance.

Adafruit_GFX.h

@@ -7,7 +7,6 @@
 #else
  #include "WProgram.h"
 #endif
-
 #include "gfxfont.h"
 
 class Adafruit_GFX : public Print {
@@ -19,16 +18,35 @@ class Adafruit_GFX : public Print {
   // This MUST be defined by the subclass:
   virtual void drawPixel(int16_t x, int16_t y, uint16_t color) = 0;
 
+  // TRANSACTION API / CORE DRAW API
+  // These MAY be overridden by the subclass to provide device-specific
+  // optimized code.  Otherwise 'generic' versions are used.
+  virtual void startWrite(void);
+  virtual void writePixel(int16_t x, int16_t y, uint16_t color);
+  virtual void writeFillRect(int16_t x, int16_t y, int16_t w, int16_t h, uint16_t color);
+  virtual void writeFastVLine(int16_t x, int16_t y, int16_t h, uint16_t color);
+  virtual void writeFastHLine(int16_t x, int16_t y, int16_t w, uint16_t color);
+  virtual void writeLine(int16_t x0, int16_t y0, int16_t x1, int16_t y1, uint16_t color);
+  virtual void endWrite(void);
+
+  // CONTROL API
+  // These MAY be overridden by the subclass to provide device-specific
+  // optimized code.  Otherwise 'generic' versions are used.
+  virtual void setRotation(uint8_t r);
+  virtual void invertDisplay(boolean i);
+
+  // BASIC DRAW API
   // These MAY be overridden by the subclass to provide device-specific
   // optimized code.  Otherwise 'generic' versions are used.
   virtual void
-    drawLine(int16_t x0, int16_t y0, int16_t x1, int16_t y1, uint16_t color),
+    // It's good to implement those, even if using transaction API
     drawFastVLine(int16_t x, int16_t y, int16_t h, uint16_t color),
     drawFastHLine(int16_t x, int16_t y, int16_t w, uint16_t color),
-    drawRect(int16_t x, int16_t y, int16_t w, int16_t h, uint16_t color),
     fillRect(int16_t x, int16_t y, int16_t w, int16_t h, uint16_t color),
     fillScreen(uint16_t color),
-    invertDisplay(boolean i);
+    // Optional and probably not necessary to change
+    drawLine(int16_t x0, int16_t y0, int16_t x1, int16_t y1, uint16_t color),
+    drawRect(int16_t x, int16_t y, int16_t w, int16_t h, uint16_t color);
 
   // These exist only with Adafruit_GFX (no subclass overrides)
   void
@@ -63,7 +81,6 @@ class Adafruit_GFX : public Print {
     setTextColor(uint16_t c, uint16_t bg),
     setTextSize(uint8_t s),
     setTextWrap(boolean w),
-    setRotation(uint8_t r),
     cp437(boolean x=true),
     setFont(const GFXfont *f = NULL),
     getTextBounds(char *string, int16_t x, int16_t y,