Optimize cellPngDecDecodeData's CELL_PNGDEC_ARGB case

Saw low hanging fruit and optimized cellPngDecDecodeData's CELL_PNGDEC_ARGB case.

With a debug build running Solar v2.1, the function went from taking 15.3% of the time from init to first frame to 0.33% of the time. Alternatively, one may optimize to 6.7% of the time with:

```
for (uint i = 0; i < image_size; i += nComponents)
{
	uint val = *(uint*)&(image.get()[i]);
	val = (val >> 24) | (val << 8); // set alpha (A8) as leftmost byte
	data.AppendRawBytes((u8*)&val, 4); 
}
```