Parallel scanning/generation, and combined scanning/preview/sprite (#820)

* Implement parallel scanning and generation, and combined scanning/preview/sprite generation.
* Added UI component for preview/sprite generation during scan, and configurable number of parallel tasks.
* Add v050 changelog entry

vendor/github.com/remeh/sizedwaitgroup/sizedwaitgroup.go

@@ -0,0 +1,84 @@
+// Based upon sync.WaitGroup, SizedWaitGroup allows to start multiple
+// routines and to wait for their end using the simple API.
+
+// SizedWaitGroup adds the feature of limiting the maximum number of
+// concurrently started routines. It could for example be used to start
+// multiples routines querying a database but without sending too much
+// queries in order to not overload the given database.
+//
+// Rémy Mathieu © 2016
+package sizedwaitgroup
+
+import (
+	"context"
+	"math"
+	"sync"
+)
+
+// SizedWaitGroup has the same role and close to the
+// same API as the Golang sync.WaitGroup but adds a limit of
+// the amount of goroutines started concurrently.
+type SizedWaitGroup struct {
+	Size int
+
+	current chan struct{}
+	wg      sync.WaitGroup
+}
+
+// New creates a SizedWaitGroup.
+// The limit parameter is the maximum amount of
+// goroutines which can be started concurrently.
+func New(limit int) SizedWaitGroup {
+	size := math.MaxInt32 // 2^32 - 1
+	if limit > 0 {
+		size = limit
+	}
+	return SizedWaitGroup{
+		Size: size,
+
+		current: make(chan struct{}, size),
+		wg:      sync.WaitGroup{},
+	}
+}
+
+// Add increments the internal WaitGroup counter.
+// It can be blocking if the limit of spawned goroutines
+// has been reached. It will stop blocking when Done is
+// been called.
+//
+// See sync.WaitGroup documentation for more information.
+func (s *SizedWaitGroup) Add() {
+	s.AddWithContext(context.Background())
+}
+
+// AddWithContext increments the internal WaitGroup counter.
+// It can be blocking if the limit of spawned goroutines
+// has been reached. It will stop blocking when Done is
+// been called, or when the context is canceled. Returns nil on
+// success or an error if the context is canceled before the lock
+// is acquired.
+//
+// See sync.WaitGroup documentation for more information.
+func (s *SizedWaitGroup) AddWithContext(ctx context.Context) error {
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case s.current <- struct{}{}:
+		break
+	}
+	s.wg.Add(1)
+	return nil
+}
+
+// Done decrements the SizedWaitGroup counter.
+// See sync.WaitGroup documentation for more information.
+func (s *SizedWaitGroup) Done() {
+	<-s.current
+	s.wg.Done()
+}
+
+// Wait blocks until the SizedWaitGroup counter is zero.
+// See sync.WaitGroup documentation for more information.
+func (s *SizedWaitGroup) Wait() {
+	s.wg.Wait()
+}