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

ssd1306/ssd1306_i2c.go

@@ -0,0 +1,52 @@
+package ssd1306
+
+import (
+	"tinygo.org/x/drivers"
+)
+
+type I2CBus struct {
+	wire    drivers.I2C
+	address uint16
+	buffer  []byte // buffer to avoid heap allocations
+}
+
+// NewI2C creates a new SSD1306 connection. The I2C wire must already be configured.
+func NewI2C(bus drivers.I2C) *Device {
+	return &Device{
+		bus: &I2CBus{
+			wire:    bus,
+			address: Address,
+		},
+	}
+}
+
+// configure address for the I2C bus and allocate the buffer
+func (b *I2CBus) configure(address uint16, size int16) []byte {
+	if address != 0 {
+		b.address = address
+	}
+	b.buffer = make([]byte, size+2) // +1 for the mode and +1 for a command
+	return b.buffer[2:]             // return the image buffer
+}
+
+// command sends a command to the display
+func (b *I2CBus) command(cmd uint8) error {
+	b.buffer[0] = 0x00 // Command mode
+	b.buffer[1] = cmd
+	return b.wire.Tx(b.address, b.buffer[:2], nil)
+}
+
+// flush sends the image to the display
+func (b *I2CBus) flush() error {
+	b.buffer[1] = 0x40 // Data mode
+	return b.wire.Tx(b.address, b.buffer[1:], nil)
+}
+
+// tx sends data to the display
+func (b *I2CBus) tx(data []byte, isCommand bool) error {
+	if isCommand {
+		return b.command(data[0])
+	}
+	copy(b.buffer[2:], data)
+	return b.flush()
+}