Cycles: support loading images from arbitrary OpenColorIO color space

These are the internal changes to Cycles, for Blender integration there are no
functional changes in this commit.

Images are converted to scene linear color space on file load, and on reading
from the OpenImageIO texture cache. 8-bit images are compressed with the sRGB
transfer function to avoid precision loss while keeping memory usages low. This
also means that for common cases of 8-bit sRGB images no conversion happens at
all on image loading.

Initial patch by Lukas, completed by Brecht.

Differential Revision: https://developer.blender.org/D3491

intern/cycles/render/shader.cpp

@@ -14,9 +14,11 @@
  * limitations under the License.
  */
 
+#include "device/device.h"
+
 #include "render/background.h"
 #include "render/camera.h"
-#include "device/device.h"
+#include "render/colorspace.h"
 #include "render/graph.h"
 #include "render/integrator.h"
 #include "render/light.h"
@@ -717,6 +719,8 @@ void ShaderManager::free_memory()
 #ifdef WITH_OSL
   OSLShaderManager::free_memory();
 #endif
+
+  ColorSpaceManager::free_memory();
 }
 
 float ShaderManager::linear_rgb_to_gray(float3 c)