add-raw-pixel-buffer-to-image: skia impl

[bnert]

Pixel buffer support has been added for the Skia backend.

By default, the memory for the image is copied over to be managed
by the SkBitmap instance within the image. This is to give the caller
safety when using a raw pointer after creating an image.

[bnert]

@djowel here is the Skia implementation, following up on our conversation on discord/cycfi/elements#227

I do not have access to a Mac/macOS at the moment. Would you, or someone else be able to handle that chunk of work?

[djowel]

Wow, you are fast! :-)

Any chance you can do something with the Quartz-2D backend? :-) I probably shouldn't be pushing my luck. Haha :-)

I'll review ASAP.

[djowel]

I do not have access to a Mac/macOS at the moment. Would you, or someone else be able to handle that chunk of work?

Ah there. Of course, I can do that.

[djowel]

Added some comments above ^. I'll look closer again later... Many thanks for your contribution!

[bnert]

For sure! Thanks for the code review!

[djowel]

Come to think of it, infra/support.hpp should probably have a to_little_endian(x) and to_big_endian(x) set of overloads for different scalar types.

[bnert]

Added an issue in cycfi/infra: cycfi/infra#12

[djowel]

Some things I have in mind, but I can add/implement later:

1. The concept of the raw pixel buffer, instead of uint8_t* data cries out for an abstraction, so you do not have to do the array management every time as it presents itself as an abstracted 2D array of pixels. It should be similar to the buffer in infra (which should really be renamed as audio_buffer or generalized to handle pixel buffers as well). I'll deal with that later.
2. It should be unified with the uint32_t const* image::pixels() member function. I'll also deal with this later, but do take note that the API as presented in this PR is subject to change.

For now, I'm good with merging this PR. Thanks for working on this!

[djowel]

BTW, I am moving this to the raw_pixel branch. It will stay there until the quartz-2d implementation is added, so as not to break the develop branch.

[bnert]

Those are good points!

Not to beat a dead branch, but as far as an abstraction, do you mean something like the pixmap class in elements?

[djowel]

Not to beat a dead branch, but as far as an abstraction, do you mean something like the pixmap class in elements?

Not really. Something like: https://github.com/cycfi/infra/blob/master/include/infra/buffer.hpp for pixmaps. So you can write things like:

pm[3][4] = pixel;

or iterate like:

for (auto& row : pm)
   for (auto& pix : row)
      pix = some_color;