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);