Tweak gamma on ST7789 and match UI colors

This change will increase the color accuracy of the PineTime and make UI
development with the simulator easier.

src/drivers/St7789.cpp

@@ -23,6 +23,7 @@ void St7789::Init() {
   RowAddressSet();
   DisplayInversionOn();
   NormalModeOn();
+  SetGamma();
   DisplayOn();
 }
 
@@ -114,6 +115,11 @@ void St7789::WriteToRam() {
   WriteCommand(static_cast<uint8_t>(Commands::WriteToRam));
 }
 
+void St7789::SetGamma() {
+  WriteCommand(static_cast<uint8_t>(Commands::GammaSet));
+  WriteData(0x04);
+}
+
 void St7789::DisplayOff() {
   WriteCommand(static_cast<uint8_t>(Commands::DisplayOff));
   nrf_delay_ms(500);

src/drivers/St7789.h

@@ -43,6 +43,7 @@ namespace Pinetime {
       void NormalModeOn();
       void WriteToRam();
       void SetAddrWindow(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1);
+      void SetGamma();
       void WriteCommand(uint8_t cmd);
       void WriteSpi(const uint8_t* data, size_t size);
 
@@ -52,6 +53,7 @@ namespace Pinetime {
         SleepOut = 0x11,
         NormalModeOn = 0x13,
         DisplayInversionOn = 0x21,
+        GammaSet = 0x26,
         DisplayOff = 0x28,
         DisplayOn = 0x29,
         ColumnAddressSet = 0x2a,