Improve texture loading

[rickgaiser]

It's now more readable and easy to add new file extensions or loaders for certain file types:

static struct STexLoader texLoader[] = {
    {"png", texPngLoad},
    {"jpg", texJpgLoad},
    {"bmp", texBmpLoad},
    {NULL, NULL}};

Add texFree function for deleting a texture. This solves some memory leaks, where for instance the texture clut was not freed.

Remove the 'psm' argument from most functions. the argument was mostly ignored and confusing. When a file exists, it is loaded, and the psm is set in the texture struct.

[KrahJohlito]

Looks good, much cleaner & works well. Thanks @rickgaiser!