3d: Add ALPHA_MAP_TEST mode

It does alpha testing using alpha map texture.

shaders/reign.f.glsl

@@ -22,6 +22,7 @@
 #define ALPHA_BLEND 0
 #define ALPHA_TEST 1
 #define ALPHA_MAP_BLEND 2
+#define ALPHA_MAP_TEST 3
 
 #if ENGINE == REIGN_ENGINE
 
@@ -180,6 +181,9 @@ void main() {
 			discard;
 	} else if (alpha_mode == ALPHA_MAP_BLEND) {
 		alpha *= texture(alpha_texture, tex_coord).r;
+	} else if (alpha_mode == ALPHA_MAP_TEST) {
+		if (texture(alpha_texture, tex_coord).r < 0.8)
+			discard;
 	}
 
 	frag_color = vec4(frag_rgb, alpha);

src/3d/renderer.c

@@ -52,6 +52,7 @@ enum {
 	ALPHA_BLEND = 0,
 	ALPHA_TEST = 1,
 	ALPHA_MAP_BLEND = 2,
+	ALPHA_MAP_TEST = 3,
 };
 
 enum draw_phase {
@@ -426,7 +427,7 @@ static void render_model(struct RE_instance *inst, struct RE_renderer *r, enum d
 		}
 
 		if (material->alpha_map) {
-			glUniform1i(r->alpha_mode, ALPHA_MAP_BLEND);
+			glUniform1i(r->alpha_mode, mesh->is_transparent ? ALPHA_MAP_BLEND : ALPHA_MAP_TEST);
 			glActiveTexture(GL_TEXTURE0 + ALPHA_TEXTURE_UNIT);
 			glBindTexture(GL_TEXTURE_2D, material->alpha_map);
 			glUniform1i(r->alpha_texture, ALPHA_TEXTURE_UNIT);