gpu: tolerate changing maximum dimensions in packer.tryAdd

This change allows atlas packer clients to start with large maximum
atlas dimensions, then shrink the maximum once the underlying atlas has
been allocated.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/pack.go

@@ -41,7 +41,7 @@ func (p *packer) newPage() {
 	p.sizes = append(p.sizes, image.Point{})
 	p.spaces = p.spaces[:0]
 	p.spaces = append(p.spaces, image.Rectangle{
-		Max: image.Point{X: p.maxDim, Y: p.maxDim},
+		Max: image.Point{X: 1e6, Y: 1e6},
 	})
 }
 
@@ -61,9 +61,15 @@ func (p *packer) tryAdd(s image.Point) (placement, bool) {
 		idx := len(p.sizes) - 1
 		size := p.sizes[idx]
 		if x := space.Min.X + s.X; x > size.X {
+			if x > p.maxDim {
+				continue
+			}
 			size.X = x
 		}
 		if y := space.Min.Y + s.Y; y > size.Y {
+			if y > p.maxDim {
+				continue
+			}
 			size.Y = y
 		}
 		if size.X*size.Y < bestSize.X*bestSize.Y {