UI: Add scene editing

So, scene editing was interesting (and by interesting I mean
excruciating).  I almost implemented 'manipulator' visuals (ala 3dsmax
for example), and used 3 modes for controlling position/rotation/size,
but in a 2D editing, it felt clunky, so I defaulted back to simply
click-and-drag for movement, and then took a similar though slightly
different looking approach for handling scaling and reszing.

I also added a number of menu item helpers related to positioning,
scaling, rotating, flipping, and resetting the transform back to
default.

There is also a new 'transform' dialog (accessible via menu) which will
allow you to manually edit every single transform variable of a scene
item directly if desired.

If a scene item does not have bounds active, pulling on the sides of a
source will cause it to resize it via base scale rather than by the
bounding box system (if the source resizes that scale will apply).  If
bounds are active, it will modify the bounding box only instead.

How a source scales when a bounding box is active depends on the type of
bounds being used.  You can set it to scale to the inner bounds, the
outer bounds, scale to bounds width only, scale to bounds height only,
and a setting to stretch to bounds (which forces a source to always draw
at the bounding box size rather than be affected by its internal size).
You can also set it to be used as a 'maximum' size, so that the source
doesn't necessarily get scaled unless it extends beyond the bounds.

Like in OBS1, objects will snap to the edges unless the control key is
pressed.  However, this will now happen even if the object is rotated or
oriented in any strange way.  Snapping will also occur when stretching
or changing the bounding box size.

libobs/obs-scene.c

@@ -175,14 +175,19 @@ static void calculate_bounds_data(struct obs_scene_item *item,
 		struct vec2 *origin, struct vec2 *scale,
 		uint32_t *cx, uint32_t *cy)
 {
-	float width         = (float)(*cx) * fabsf(scale->x);
-	float height        = (float)(*cy) * fabsf(scale->y);
-	float item_aspect   = width / height;
-	float bounds_aspect = item->bounds.x / item->bounds.y;
-	float width_diff, height_diff;
+	float    width         = (float)(*cx) * fabsf(scale->x);
+	float    height        = (float)(*cy) * fabsf(scale->y);
+	float    item_aspect   = width / height;
+	float    bounds_aspect = item->bounds.x / item->bounds.y;
+	uint32_t bounds_type   = item->bounds_type;
+	float    width_diff, height_diff;
 
-	if (item->bounds_type == OBS_BOUNDS_SCALE_INNER ||
-	    item->bounds_type == OBS_BOUNDS_SCALE_OUTER) {
+	if (item->bounds_type == OBS_BOUNDS_MAX_ONLY)
+		if (width > item->bounds.x || height > item->bounds.y)
+			bounds_type = OBS_BOUNDS_SCALE_INNER;
+
+	if (bounds_type == OBS_BOUNDS_SCALE_INNER ||
+	    bounds_type == OBS_BOUNDS_SCALE_OUTER) {
 		bool  use_width = (bounds_aspect < item_aspect);
 		float mul;
 
@@ -195,13 +200,13 @@ static void calculate_bounds_data(struct obs_scene_item *item,
 
 		vec2_mulf(scale, scale, mul);
 
-	} else if (item->bounds_type == OBS_BOUNDS_SCALE_TO_WIDTH) {
+	} else if (bounds_type == OBS_BOUNDS_SCALE_TO_WIDTH) {
 		vec2_mulf(scale, scale, item->bounds.x / width);
 
-	} else if (item->bounds_type == OBS_BOUNDS_SCALE_TO_HEIGHT) {
+	} else if (bounds_type == OBS_BOUNDS_SCALE_TO_HEIGHT) {
 		vec2_mulf(scale, scale, item->bounds.y / height);
 
-	} else if (item->bounds_type == OBS_BOUNDS_STRETCH) {
+	} else if (bounds_type == OBS_BOUNDS_STRETCH) {
 		scale->x = item->bounds.x / (float)(*cx);
 		scale->y = item->bounds.y / (float)(*cy);
 	}

libobs/obs.h

@@ -95,12 +95,12 @@ enum allow_direct_render {
  */
 enum obs_bounds_type {
 	OBS_BOUNDS_NONE,            /**< no bounds */
-	OBS_BOUNDS_ALIGN_ONLY,      /**< no scaling to bounds, align only */
+	OBS_BOUNDS_STRETCH,         /**< stretch (ignores base scale) */
 	OBS_BOUNDS_SCALE_INNER,     /**< scales to inner rectangle */
 	OBS_BOUNDS_SCALE_OUTER,     /**< scales to outer rectangle */
 	OBS_BOUNDS_SCALE_TO_WIDTH,  /**< scales to the width  */
 	OBS_BOUNDS_SCALE_TO_HEIGHT, /**< scales to the height */
-	OBS_BOUNDS_STRETCH          /**< stretch (ignores base scale) */
+	OBS_BOUNDS_MAX_ONLY,        /**< no scaling, maximum size only */
 };
 
 struct obs_sceneitem_info {