Fix broken colors and partial flush for ILI9488 display

src/nanoFramework.Graphics/Graphics/Displays/ILI9488_480x320_SPI.cpp

@@ -289,15 +289,17 @@ void DisplayDriver::BitBlt(
 
     g_DisplayInterface.SendCommand(1, Memory_Write);
 
-    uint32_t numPixels = width * height;
     uint32_t count = 0;
 
     CLR_UINT8 *TransferBuffer = Attributes.TransferBuffer;
     CLR_UINT32 TransferBufferSize = Attributes.TransferBufferSize;
 
     // only 18/24 bit is supported on SPI
-    for (uint32_t i = 0; i < numPixels; i++)
+    for (uint32_t y = srcY; y < srcY + height; y++)
+    for (uint32_t x = srcX; x < srcX + width; x++)
     {
+        uint32_t i = y * Attributes.Width + x;
+
         uint32_t element = data[i / 2]; // Each uint32 stores 2 pixels
         uint16_t color = (i % 2 == 0) ? (element & 0xFFFF) : (element >> 16);
 
@@ -309,9 +311,9 @@ void DisplayDriver::BitBlt(
         g = (g << 2) | (g >> 4);
         r = (r << 3) | (r >> 2);
 
-        TransferBuffer[count++] = b;
-        TransferBuffer[count++] = g;
         TransferBuffer[count++] = r;
+        TransferBuffer[count++] = g;
+        TransferBuffer[count++] = b;
 
         // can't fit another 3 bytes
         if (count + 3 > TransferBufferSize - 1)