A variation is a #define/specialization constant that every shader can
use to specialize itself as it sees fit.
This commit adds the infrastrcture, future commits will add
implementations.
This makes the (currently single) storage buffer handled by
GskGpuDescriptors.
A side effect is that we now have support for multiple buffers in place.
We just have to use it.
Mixed into this commit is a complete rework of the pattern writer.
Instead of writing straight into the buffer (complete with repeatedly
backtracking when we have to do offscreens), write into a temporary
buffer and copy into the storage buffer on committing.
Carry an n_external_textures variable around when selecting programs and
compile different programs for different amounts of external textures.
For now, this code is unused, but dmabufs will need it.
Introduce a new GskGpuImageDescriptors object that tracks descriptors
for a set of images that can be managed by the GPU.
Then have each GskGpuShaderOp just reference the descriptors object they are
using, so that the coe can set things up properly.
To reference an image, the ops now just reference their descriptor -
which is the uint32 we've been sending to the shaders since forever.
Use glDrawArraysInstancedBaseInstance() to draw. (Yay for GL naming.)
That allows setting up the offset in the vertex array without having to
glVertexAttribPointer() everything again.
However, this is only supported since GL 4.2 and not at all in stock GLES,
so we need to have code that can work without it.
Fortunately, it is mandatory in Vulkan, so every recent GPU supports it.
And if that GPU has a proper driver, it will also expose the GL extension
for it.
(Hint: You can check https://opengles.gpuinfo.org/listextensions.php for
how many proper drivers exist outside of Mesa.)
Because GL flips its shit sometimes (ie when it's the framebuffer),
pass the height of the target as the flip variable, so commands
that need to operate on the pixels can flip the y axis around this value.
This heaves over an inital chunk of code from the Vulkan renderer to
execute shaders.
The only shader that exists for now is a shader that draws a single
texture.
We use that to replace the blit op we were doing before.
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
