Merge gyp files into panda/panda.gyp.
Split scene into scene/layers and drawables.
Move panda_layer_add_line to draw.c.
Rename gl_draw -> gl_draw_scene.
Encapsulate line into panda_drawable.

Avoids using panda_line directly in input.c or layers which makes scene
management draw-type agnostic.
Draw layer-content boundaries.
Make sure a line is present when updating cursor.

Fixes assertion crash triggering when drawing a line, e.g. releasing all
modifier keys, while zooming or panning.