cute_aseprite: Add chunck 0x0004 support

[redbug26]

With last version of aseprite (v1.3.2), when we save a file where no alpha is used in the palette, only the chunk 0x0004 is created (not the 0x2019).
0x0004 chunk support is then needed

[redbug26]

From doc:

Old palette chunk (0x0004): Ignore this chunk if you find the new palette chunk (0x2019) Aseprite v1.1 saves both chunks 0x0004 and 0x2019 just for backward compatibility.

This chunk is thus mandatory if chunk 2019 is not available.

From source:

Chunk 0x2019 is only available if:
if (pal->size() != 256 || pal->hasAlpha()) {

[RobLoach]

Getting one warning when strict compiling....

cute_aseprite.h: In function ‘cute_aseprite_load_from_memory’:
cute_aseprite.h:1014:83: warning: conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} may change value [-Wconversion]
 1014 |                                                 if (skip+l > maxColor) maxColor = skip+l;