cute_aseprite.h: Added support for indexed and grayscale pixels

[mathewmariani]

This pull requests adds support for color modes grayscale and indexed color modes.

struct ase_frame_t now has a field void* pixels; instead of a ase_color_t* pixels to reflect how the specs defined a pixel. We can cast to our desired color types (struct ase_color_t, struct ase_grayscale_t, and struct ase_index) as needed.

A pixel as described in the specs:

* `PIXEL`: One pixel, depending on the image pixel format:
  - **RGBA**: `BYTE[4]`, each pixel have 4 bytes in this order Red, Green, Blue, Alpha.
  - **Grayscale**: `BYTE[2]`, each pixel have 2 bytes in the order Value, Alpha.
  - **Indexed**: `BYTE`, each pixel uses 1 byte (the index).

Blending is not supported for ASE_MODE_GRAYSCALE and ASE_MODE_INDEXED and s_blend will only be called when the color mode is ASE_MODE_RGBA.

[RobLoach]

Finding this?

cute_aseprite.h:1279:75: error: ‘dst_index’ undeclared (first use in this function); did you mean ‘tag_index’?
 1279 |                                       ase_color_t dst_color = d[dst_index];
      |                                                                 ^~~~~~~~~
      |                                                                 tag_index
cute_aseprite.h:1279:75: note: each undeclared identifier is reported only once for each function it appears in

After fixing that, I'm still getting no rendered image.

[RandyGaul]

Need to match formatting style for the rest of the file -- (ase_color_t*) not ase_color_t *

[RandyGaul]

Formatting: ((ase_grayscale_t *) src) -> ((ase_grayscale_t*)src)

[RandyGaul]

We need to leave a helper function so people can add in their own new blending support. The blending loops below are optional and only added in for convenience. There are many ways to blend aseprite layers together, I just picked the most "typical" to support for now.

[RandyGaul]

Did you accidentally delete this?

[RandyGaul]

I'd make a little more of a fleshed out abstraction for rendering different kinds of images. Kind of like how I had a helper function to fetch pixels of different types, you should probably invest some time to write something to help you write pixels of different types for different image output formats. This will make the code much more reusable for implementing different kinds of layer blending.