Group Adapters by their categories

Resolves https://github.com/pion/mediadevices/issues/22

Created 2 new directories under driver, camera and microphone. Moved
camera_linux.go and microphone_linux.go appropriately. Imported
driver/camera and driver/microphone by default.

pkg/driver/microphone/microphone_linux.go

@@ -0,0 +1,109 @@
+package microphone
+
+import (
+	"io"
+	"time"
+
+	"github.com/jfreymuth/pulse"
+	"github.com/pion/mediadevices/pkg/driver"
+	"github.com/pion/mediadevices/pkg/io/audio"
+	"github.com/pion/mediadevices/pkg/prop"
+)
+
+type microphone struct {
+	c           *pulse.Client
+	samplesChan chan<- []float32
+}
+
+func init() {
+	driver.GetManager().Register(&microphone{})
+}
+
+func (m *microphone) Open() error {
+	var err error
+	m.c, err = pulse.NewClient()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (m *microphone) Close() error {
+	if m.samplesChan != nil {
+		close(m.samplesChan)
+		m.samplesChan = nil
+	}
+
+	m.c.Close()
+	return nil
+}
+
+func (m *microphone) AudioRecord(p prop.Media) (audio.Reader, error) {
+	var options []pulse.RecordOption
+	if p.ChannelCount == 1 {
+		options = append(options, pulse.RecordMono)
+	} else {
+		options = append(options, pulse.RecordStereo)
+	}
+	latency := p.Latency.Seconds()
+	options = append(options, pulse.RecordSampleRate(p.SampleRate), pulse.RecordLatency(latency))
+
+	samplesChan := make(chan []float32, 1)
+	var buff []float32
+	var bi int
+	var more bool
+
+	handler := func(b []float32) {
+		samplesChan <- b
+	}
+
+	stream, err := m.c.NewRecord(handler, options...)
+	if err != nil {
+		return nil, err
+	}
+
+	reader := audio.ReaderFunc(func(samples [][2]float32) (n int, err error) {
+		for i := range samples {
+			// if we don't have anything left in buff, we'll wait until we receive
+			// more samples
+			if bi == len(buff) {
+				buff, more = <-samplesChan
+				if !more {
+					stream.Close()
+					return i, io.EOF
+				}
+				bi = 0
+			}
+
+			samples[i][0] = buff[bi]
+			if p.ChannelCount == 2 {
+				samples[i][1] = buff[bi+1]
+				bi++
+			}
+			bi++
+		}
+
+		return len(samples), nil
+	})
+
+	stream.Start()
+	m.samplesChan = samplesChan
+	return reader, nil
+}
+
+func (m *microphone) Properties() []prop.Media {
+	// TODO: Get actual properties
+	monoProp := prop.Media{
+		Audio: prop.Audio{
+			SampleRate:   48000,
+			Latency:      time.Millisecond * 20,
+			ChannelCount: 1,
+		},
+	}
+
+	stereoProp := monoProp
+	stereoProp.ChannelCount = 2
+
+	return []prop.Media{monoProp, stereoProp}
+}