feat(examples): add shading, the threaded render loop and two scenes

Lambertian and metal shading with recursive bounces, a row-claiming loop
across worker threads, and the studio and rings scene files.

examples/raytracer/scenes/rings.scene

@@ -0,0 +1,17 @@
+# A ring of alternating metal and diffuse spheres around a central emitter.
+
+camera     0 3.2 8.0   0 0.6 0   30
+background 0.010 0.012 0.020
+
+sphere  0.0 -500.0  0.0  500.0  lambert  0.55 0.55 0.58
+
+sphere  2.60  0.55  0.00   0.55  metal    0.95 0.80 0.45  0.02
+sphere  1.84  0.55  1.84   0.55  lambert  0.85 0.30 0.30
+sphere  0.00  0.55  2.60   0.55  metal    0.85 0.85 0.90  0.10
+sphere -1.84  0.55  1.84   0.55  lambert  0.30 0.75 0.45
+sphere -2.60  0.55  0.00   0.55  metal    0.60 0.70 0.95  0.02
+sphere -1.84  0.55 -1.84   0.55  lambert  0.80 0.55 0.20
+sphere  0.00  0.55 -2.60   0.55  metal    0.90 0.90 0.90  0.00
+sphere  1.84  0.55 -1.84   0.55  lambert  0.45 0.35 0.80
+
+sphere  0.00  1.10  0.00   0.70  light    4.20 3.60 2.60

examples/raytracer/scenes/studio.scene

@@ -0,0 +1,13 @@
+# Three spheres on a ground plane, lit by one emissive sphere overhead.
+
+camera     0 1.4 7.5   0 0.7 0   34
+background 0.020 0.028 0.045
+
+#      centre            radius  material  albedo              fuzz
+sphere  0.0 -500.0  0.0   500.0  lambert   0.62 0.62 0.60
+sphere -2.1  0.75   0.1     0.75 lambert   0.86 0.24 0.22
+sphere  0.0  0.85  -0.3     0.85 metal     0.92 0.88 0.78   0.03
+sphere  2.1  0.70   0.2     0.70 lambert   0.22 0.42 0.86
+sphere  0.6  0.32   1.5     0.32 metal     0.75 0.75 0.80   0.28
+sphere -0.9  0.28   1.7     0.28 lambert   0.90 0.72 0.20
+sphere  0.0  6.20   1.0     3.20 light     3.60 3.40 3.00