Uber-pass frame rendering

[kvark]

WebRender is currently getting support for Pixel Local Storage GL extension. The idea is to take advantage of super-fast tile storage on mobile (and future Intel) devices. The Vulkan way of doing this would be with sub-passes.

WebRender frame consists of a number of passes, each with multiple layers of work. In order for the driver to figure out the best tile memory allocation/lifetime, it needs to know about all the dependent passes. So ideally, we'd just provide each slice of each participating texture as a framebuffer attachment, and the rendering to a slice would be turned into a separate sub-pass. We may very well end up with a hundred-ish attachments/sub-passes, hence the "uber-pass" name.

All of the intermediate render targets just need to be turned into transient attachments, since their lifetime is completely contained in the uber-pass. What is important for WebRender is to properly communicate the dependencies: we'll need to specify a flag to gfx-rs when an input attachment is sampled with precisely the same coordinate as the output - that would allow the driver to keep the intermediate value in the tile cache. Marking those dependencies, as well as re-arranging the work to make more of those, could be done incrementally after the main uber-pass is introduced.

There doesn't appear to be limits in Vulkan/gfx-rs preventing us to do this, and the driver can always fall back to flushing each individual sub-pass results to VRAM, as it would do on current desktop GPUs anyway.