Expose PreviewImage & custom icons to py API.

This commit mainly:

* Exposes PreviewImage struct in RNA, including ways for user to set images data.
* Adds a new kind of PreviewImage, using a file path and IMB_thumb to get image.
* Adds a new kind of custom icon using PreviewImage, unrelated to ID previews system.
* Adds a python API (utils.previews) to allow python scripts to access those custom previews/icons.

Note that loading image from files' thumbnails is done when needed (deferred loading), not
when defining the custom preview/icon.

WARNING: for release addons who would want to use this, please keep it to a strict minimum, really needed level.
We do not want our UI to explode under hundreds of different flashy icons!

For more info, see also the release notes of Blender 2.75 (http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.75/Addons)
and the example/templates featured with Blender.

Patch by Campbell (ideasman42), Inês (brita) and Bastien (mont29).

Differential Revision: https://developer.blender.org/D1255

1 files changed, 19 insertions, 3 deletions

diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index b51b53cc801..f68de03614b 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -155,20 +155,36 @@ typedef struct Library {
 
 enum eIconSizes {
 	ICON_SIZE_ICON = 0,
-	ICON_SIZE_PREVIEW = 1
+	ICON_SIZE_PREVIEW = 1,
+
+	NUM_ICON_SIZES
+};
+
+/* for PreviewImage->flag */
+enum ePreviewImage_Flag {
+	PRV_CHANGED          = (1 << 0),
+	PRV_USER_EDITED      = (1 << 1),  /* if user-edited, do not auto-update this anymore! */
 };
-#define NUM_ICON_SIZES (ICON_SIZE_PREVIEW + 1)
 
 typedef struct PreviewImage {
 	/* All values of 2 are really NUM_ICON_SIZES */
 	unsigned int w[2];
 	unsigned int h[2];
-	short changed[2];
+	short flag[2];
 	short changed_timestamp[2];
 	unsigned int *rect[2];
+
+	/* Runtime-only data. */
 	struct GPUTexture *gputexture[2];
+	int icon_id;  /* Used by previews outside of ID context. */
+
+	char pad[3];
+	char use_deferred;  /* for now a mere bool, if we add more deferred loading methods we can switch to bitflag. */
 } PreviewImage;
 
+#define PRV_DEFERRED_DATA(prv) \
+	(CHECK_TYPE_INLINE(prv, PreviewImage *), BLI_assert((prv)->use_deferred), (void *)((prv) + 1))
+
 /**
  * Defines for working with IDs.
  *