support generating plane geometry.

maybe such objects should be optimized? it's quite expensive to draw
such simple things. or maybe buffering the draw calls would help. not
sure.

js/pixl/public/trixl.html

@@ -372,6 +372,22 @@
			}
		}
		trixl.geometry.plane = function(pos, v, w, width, height, color) {
			var v = vec3.normalize([], v), w = vec3.normalize([], w);
			for (var i = 0; i < width; i++) {
				for (var j = 0; j < height; j++) {
					var pos = vec3.create();
					vec3.scaleAndAdd(pos, pos, v, i);
					vec3.scaleAndAdd(pos, pos, w, j);
					trixl.world.set([
						Math.round(pos[0]), Math.round(pos[1]), Math.round(pos[2])
					], {color: color || trixl.color});
				}
			}
		}
		var angle = {x: 0, y: 0, z: 0};
		var offset = {x: 0, y: 0, z: 0};
		program.transform = gl.getUniformLocation(program, 'transform');