ssd1306: avoid unnecessary heap allocations (#767)

* ssd1306: avoid unnecessary heap allocations

* ssd1306: extract i2c and spi bus implementations

* ssd1306: refactor tests -- show fps and heap usage

* ssd1306: bring back the lost exported methods

* Adjust examples

* Fix smoketests for ssd1306

examples/ssd1306/main.go

@@ -0,0 +1,59 @@
+package main
+
+// This example shows how to use SSD1306 OLED display driver over I2C and SPI.
+//
+// Check the `newSSD1306Display()` functions for I2C and SPI initializations.
+
+import (
+	"runtime"
+
+	"image/color"
+	"time"
+)
+
+func main() {
+
+	display := newSSD1306Display()
+	display.ClearDisplay()
+
+	w, h := display.Size()
+	x := int16(0)
+	y := int16(0)
+	deltaX := int16(1)
+	deltaY := int16(1)
+
+	traceTime := time.Now().UnixMilli() + 1000
+	frames := 0
+	ms := runtime.MemStats{}
+
+	for {
+		pixel := display.GetPixel(x, y)
+		c := color.RGBA{255, 255, 255, 255}
+		if pixel {
+			c = color.RGBA{0, 0, 0, 255}
+		}
+		display.SetPixel(x, y, c)
+		display.Display()
+
+		x += deltaX
+		y += deltaY
+
+		if x == 0 || x == w-1 {
+			deltaX = -deltaX
+		}
+
+		if y == 0 || y == h-1 {
+			deltaY = -deltaY
+		}
+
+		frames++
+		now := time.Now().UnixMilli()
+		if now >= traceTime {
+			runtime.ReadMemStats(&ms)
+			println("TS", now, "| FPS", frames, "| HeapInuse", ms.HeapInuse)
+			traceTime = now + 1000
+			frames = 0
+		}
+	}
+
+}