Fix T51412: Instant crash with texture plugged into the Displacement output

The issue was caused by unlimited textures commit, root of the issue is that
displacement code updates some of the image slots directly, so it needs to
ensure device vectors are all proper size.

intern/cycles/render/image.cpp

@@ -949,16 +949,8 @@ void ImageManager::device_free_image(Device *device, DeviceScene *dscene, ImageD
 	}
 }
 
-void ImageManager::device_update(Device *device,
-                                 DeviceScene *dscene,
-                                 Scene *scene,
-                                 Progress& progress)
+void ImageManager::device_prepare_update(DeviceScene *dscene)
 {
-	if(!need_update)
-		return;
-
-	TaskPool pool;
-
 	for(int type = 0; type < IMAGE_DATA_NUM_TYPES; type++) {
 		switch(type) {
 			case IMAGE_DATA_TYPE_FLOAT4:
@@ -986,6 +978,23 @@ void ImageManager::device_update(Device *device,
 					dscene->tex_half_image.resize(tex_num_images[IMAGE_DATA_TYPE_HALF]);
 				break;
 		}
+	}
+}
+
+void ImageManager::device_update(Device *device,
+                                 DeviceScene *dscene,
+                                 Scene *scene,
+                                 Progress& progress)
+{
+	if(!need_update) {
+		return;
+	}
+
+	/* Make sure arrays are proper size. */
+	device_prepare_update(dscene);
+
+	TaskPool pool;
+	for(int type = 0; type < IMAGE_DATA_NUM_TYPES; type++) {
 		for(size_t slot = 0; slot < images[type].size(); slot++) {
 			if(!images[type][slot])
 				continue;

intern/cycles/render/image.h

@@ -59,6 +59,7 @@ public:
 	                      bool use_alpha);
 	ImageDataType get_image_metadata(const string& filename, void *builtin_data, bool& is_linear);
 
+	void device_prepare_update(DeviceScene *dscene);
 	void device_update(Device *device,
 	                   DeviceScene *dscene,
 	                   Scene *scene,

intern/cycles/render/mesh.cpp

@@ -1944,6 +1944,7 @@ void MeshManager::device_update_displacement_images(Device *device,
 			}
 		}
 	}
+	image_manager->device_prepare_update(dscene);
 	foreach(int slot, bump_images) {
 		pool.push(function_bind(&ImageManager::device_update_slot,
 		                        image_manager,