Add outer alloc memory buffer options.

[HassenLin]

few code add for outer alloc image for low memory device.

Add outer_alloc flag.
Add int quirc_set_image_buffer(struct quirc* q, uint8_t* image_buffer);
call quirc_set_image_buffer(struct quirc* q, uint8_t* image_buffer); before int quirc_resize(struct quirc *q, int w, int h);
tested on esp32s2 (esp-idf 4.2)

[dlbeer]

It looks like this code assumes that quirc_pixel_t is uint8_t? It may be different depending on compile-time options, so that needs to be taken into account in any allocations, and probably also in the function prototypes.