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]
}

libobs/graphics/effect.c

@@ -50,6 +50,46 @@ gs_technique_t *gs_effect_get_current_technique(const gs_effect_t *effect)
 	return effect->cur_technique;
 }
 
+bool gs_effect_loop(gs_effect_t *effect, const char *name)
+{
+	if (!effect) {
+		return false;
+	}
+
+	if (!effect->looping) {
+		gs_technique_t *tech;
+
+		if (!!gs_get_effect()) {
+			blog(LOG_WARNING, "gs_effect_loop: An effect is "
+			                  "already active");
+			return false;
+		}
+
+		tech = gs_effect_get_technique(effect, name);
+		if (!tech) {
+			blog(LOG_WARNING, "gs_effect_loop: Technique '%s' "
+			                  "not found.", name);
+			return false;
+		}
+
+		gs_technique_begin(tech);
+
+		effect->looping = true;
+	} else {
+		gs_technique_end_pass(effect->cur_technique);
+	}
+
+	if (!gs_technique_begin_pass(effect->cur_technique,
+				effect->loop_pass++)) {
+		gs_technique_end(effect->cur_technique);
+		effect->looping = false;
+		effect->loop_pass = 0;
+		return false;
+	}
+
+	return true;
+}
+
 size_t gs_technique_begin(gs_technique_t *tech)
 {
 	if (!tech) return 0;

libobs/graphics/effect.h

@@ -147,6 +147,9 @@ struct gs_effect {
 
 	gs_eparam_t *view_proj, *world, *scale;
 	graphics_t *graphics;
+
+	size_t loop_pass;
+	bool looping;
 };
 
 static inline void effect_init(gs_effect_t *effect)

libobs/graphics/graphics.h

@@ -358,6 +358,11 @@ EXPORT gs_eparam_t *gs_effect_get_param_by_idx(const gs_effect_t *effect,
 EXPORT gs_eparam_t *gs_effect_get_param_by_name(const gs_effect_t *effect,
 		const char *name);
 
+/** Helper function to simplify effect usage.  Use with a while loop that
+ * contains drawing functions.  Automatically handles techniques, passes, and
+ * unloading. */
+EXPORT bool gs_effect_loop(gs_effect_t *effect, const char *name);
+
 /** used internally */
 EXPORT void gs_effect_update_params(gs_effect_t *effect);