Docs: Add design doc and implementation plan for highdpi

I have a local branch that gets us past implementation stage 1
in this doc, and it seems like a useful enough checkpoint to
upstream it and then iterate in tree.

Documentation/HighDPI.md

@@ -0,0 +1,37 @@
+HighDPI design
+==============
+
+Background
+----------
+
+- macOS: Only integer scale factors at app level. Can stretch final composited framebuffer at very end for non-integer display scales.
+ - advantages: simple programming model, still fairly flexible; scaling at end produces coherent final image
+ - disadvantages: needs 4x memory even for 1.5x scale; scaling at very makes final image less detailed than it could be
+
+- Android: Many (but discrete) scale levels (ldpi (0.75x), mdpi, hdpi (1.5x), xhdpi (2x), xxhdpi (3x), xxhdpi (4x))
+
+- Windows: has "not recommended" free form text entry for scale factor between 100% and 500%.
+
+Integer scale factors are needed in any case so let's get that working first. Actually, let's focus on just 2x for now.
+
+
+Desired end state
+-----------------
+
+- Window and Widget rects are in "logical" coordinates, which is the same as pixels at 1x scale
+- Same for mouse cursor
+- Jury's still out if logical coordinates should stay ints. Probably, but it means mouse cursor etc only have point resolution, not pixel resolution
+- We should have something that can store a collection of (lazily-loaded?) bitmaps and fonts that each represent a single image / font at different scale levels, and at paint time the right representation is picked for the current scale
+
+Implementation plan
+-------------------
+
+The plan is to have all applications use highdpi backbuffers eventually. It'll take some time to get there though, so here's a plan for getting there incrementally.
+
+0. Add some scaling support to Painter. Make it do 2x nearest neighbor scaling of everything at paint time for now.
+1. Add scale factor concept to WindowServer and let DisplaySettings toggle it. WindowServer has a scaled framebuffer/backbuffer. All other bitmaps (both other bitmaps in WindowServer, as well as everything WindowServer-client-side) are always stored at 1x and scaled up when they're painted to the framebuffer. Things will look fine at 2x, but pixely (but window gradients will be smooth already).
+2. Come up with a system to have scale-dependent bitmap and font resources. Use that to use a high-res cursor bitmaps and high-res menu bar text painting in window server. Menu text and cursor will look less pixely. (And window frames too, I suppose.)
+3. Let apps opt in to high-res window framebuffers, and convert all apps one-by-one
+4. Remove high-res window framebuffer opt-in since all apps have it now.
+
+We're currently before point 0.