libobs: Add function to get cur. effect technique
Revert "Add obs_source_draw helper function"

This reverts commit ab5a76901bae7feb9d0404c7b14046a2c74e8ff6.
Add helper functions for drawing sources

If you look at the previous commits, you'll see I had added
obs_source_draw before.  For custom drawn sources in particular, each
time obs_source_draw was called, it would restart the effect and its
passes for each draw call, which was not optimal.  It should really use
the effect functions for that.  I'll have to add a function to simplify
effect usage.

I also realized that including the color matrix parameters in
obs_source_draw made the function kind of messy to use; instead,
separating the color matrix stuff out to
obs_source_draw_set_color_matrix feels a lot more clean.

On top of that, having the ability to set the position would be nice to
have as well, rather than having to mess with the matrix stuff each
time, so I also added that for the sake of convenience.

obs_source_draw will draw a texture sprite, optionally of a specific
size and/or at a specific position, as well as optionally inverted.  The
texture used will be set to the 'image' parameter of whatever effect is
currently active.

obs_source_draw_set_color_matrix will set the color matrix value if the
drawing requires color matrices.  It will set the 'color_matrix',
'color_range_min', and 'color_range_max' parameters of whatever effect
is currently active.

Overall, these feel much more clean to use than the previous iteration.
graphics: Add gs_effect_loop helper function

This function greatly simplifies the use of effects by making it so you
can call this function in a simple loop.  This reduces boilerplate and
makes drawing with effects much easier.  The gs_effect_loop function
will now automatically handle all the functions required to do drawing.

---------------------
Before:

gs_technique_t *technique = gs_effect_get_technique("technique");

size_t passes = gs_technique_begin(technique);
for (size_t pass = 0; pass < passes; pass++) {
	gs_technique_begin_pass(technique, pass);

	[draw]

	gs_technique_end_pass(technique);
}
gs_technique_end(technique);

---------------------
After:

while (gs_effect_loop(effect, "technique")) {
	[draw]
}