Fix large blacklevel step. Lower 25% of shades are now accessible.

There is a large step in brightness from level zero to level one.
After experimenting with various ST7789 options, I found that
decreasing VDV to 0x10 (-0.4V) fixes this issue.

The gamma change reduced the average error in brightness, but with the
underlying issue fixed, the gamma change has been reverted.

src/drivers/St7789.cpp

@@ -23,7 +23,7 @@ void St7789::Init() {
   RowAddressSet();
   DisplayInversionOn();
   NormalModeOn();
-  SetGamma();
+  SetVdv();
   DisplayOn();
 }
 
@@ -115,9 +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::SetVdv() {
+  // By default there is a large step from pixel brightness zero to one.
+  // After experimenting with VCOMS, VRH and VDV, this was found to produce good results.
+  WriteCommand(static_cast<uint8_t>(Commands::VdvSet));
+  WriteData(0x10);
 }
 
 void St7789::DisplayOff() {

src/drivers/St7789.h

@@ -43,7 +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 SetVdv();
       void WriteCommand(uint8_t cmd);
       void WriteSpi(const uint8_t* data, size_t size);
 
@@ -53,7 +53,6 @@ namespace Pinetime {
         SleepOut = 0x11,
         NormalModeOn = 0x13,
         DisplayInversionOn = 0x21,
-        GammaSet = 0x26,
         DisplayOff = 0x28,
         DisplayOn = 0x29,
         ColumnAddressSet = 0x2a,
@@ -63,6 +62,7 @@ namespace Pinetime {
         VerticalScrollDefinition = 0x33,
         VerticalScrollStartAddress = 0x37,
         ColMod = 0x3a,
+        VdvSet = 0xc4,
       };
       void WriteData(uint8_t data);
       void ColumnAddressSet();