linear will average all the pixels for the lod, nearest will just pick
one (using the same method as OpenGL/Vulkan, picking bottom right
center).
This doesn't really make linear/nearest filtering work as it should
(because it's still a form of mipmaps), but it has 2 advantages:
1. it gets closer to the desired effect
2. it is a lot faster
Because only 1 pixel is chosen from the original image, instead of
averaging all pixels, a lot less memory needs to be accessed, and
because memory access is the bottleneck for large images, the speedup is
almost linear with the number of pixels not accessed.
And that means that even for lot level 3, aka 1/8th scale, only 1/64 of
the pixels need to be accessed, and everything is 50x faster.
Switching gtk4-demo --run=image_scaling to linear/nearest makes all the
lag go away for me, even with a 64k x 64k image.
This allows uploading less memory but requires computing lod levels on
the CPU which is slow because it reads through all of the memory and so
far entirely not optimized.
However, it uses significantly less VRAM.
This is done by adding a gdk_memory_mipmap() function that does this
task.
The texture upload op now accepts a lod level and if that is >0 it uses
gdk_memory_mipmap() on the source texture.
This adds GSK_GPU_IMAGE_CAN_MIPMAP and GSK_GPU_IMAGE_MIPMAP flags and
support to ensure_image() and image creation functions for creating a
mipmapped image.
Mipmaps are created using the new mipmap op that uses
glGenerateMipmap() on GL and equivalent blit ops on Vulkan.
This is then used to ensure the image is mipmapped when rendering it
with a texture-scale node.
The main reason here is that we want to not fail when the texture size
is larger than the supported GpuImage size.
When that happens, for now we just fallback slowly - ulitmately to
drawing with Cairo, which is going to be clipped.
For now, it just renders using cairo, uploads the result to the GPU,
blits it onto the framebuffer and then is happy.
But it can do that using Vulkan and using GL (no idea which version).
The most important thing still missing is shaders.
It also has a bunch of copy/paste from the Vulkan renderer that isn't
used yet.
But I didn't want to rip it out and then try to copy it back later
