Scenes: Implement more item positioning features

There are a ridiculous number of features related to scaling and
positioning due to requests by a number of people who complained that
they hated the way that OBS1 would always resize their sources when the
source's base size changed.  There were also people who wanted more
control for how the resizing was handled, or the ability to completely
prevent resizing entirely if desired.  So I made it so that you can
optionally use a 'bounds' system, which allows you to specify different
styles of controlling resizing.

If disabled, the source will always automatically resize and only the
base scale is applied.  If enabled, you have a variety of different ways
to limit/control how it can resize within the bounds, or make it so it
can't resize at all.  You can also control alignment within that
bounding box, so you can make it so that a source always aligns to a
side or corner of the box.

I also added an alignment value which changes how the source is oriented
relative to the position of the scene item.  For example, setting
bottom-right alignment will make it so that the position of the item is
the bottom right corner of the source.  When the source resizies, it
will resize leftward and upward in that case, which solves the problem
of how a source resizes relative to a desired position.

libobs/obs-defs.h

@@ -20,6 +20,12 @@
 /** Maximum number of source channels for output and per display */
 #define MAX_CHANNELS 64
 
+#define OBS_ALIGN_CENTER (0)
+#define OBS_ALIGN_LEFT   (1<<0)
+#define OBS_ALIGN_RIGHT  (1<<1)
+#define OBS_ALIGN_TOP    (1<<2)
+#define OBS_ALIGN_BOTTOM (1<<3)
+
 #define MODULE_SUCCESS             0
 #define MODULE_ERROR              -1
 #define MODULE_FILE_NOT_FOUND     -2