tile mode for hi-res rendering

[cyrush]

add tile-based camera option for very high resolution renders (16k^2+)

[cyrush]

Two use cases:

1. One camera, many render passes to create 1 image

2. One camera, many (tiled) images

possible input for case 1:

renders:
    my_render:
        image_width: 32768
        image_height: 32768
        tile_width:    8192
        tile_height:  8192
        image_name: "my_image"

Renders 16 images to create one 32K x 32K image

(image_name? implies combine tiles into one png)

possible input for case 2:

renders:
    my_render:
        image_width: 32768
        image_height: 32768
        tile_width:    8192
        tile_height:  8192
        tile_name_pattern: "my_images_%04d"

(tile_name_pattern? implies separate files, one png per tile)