fix(koreader): open the dashboard on reMarkable's 16-bit screen

Draw dashboard thumbnails in RGB32 when the screen is RGB565. KOReader's
MuPDF scaler throws on RGB565, which crashed the first-run layout picker
and the widget previews on every reMarkable running under AppLoad.

Closes #1909

koreader-plugin/bookorbit.koplugin/bookorbit_board_ui.lua

@@ -856,7 +856,11 @@ function BoardUI.snapshot(key, full_w, full_h, w, h, build)
     if not snapshot_cache[key] then
         local Screen = require("device").screen
         local Blitbuffer_ = require("ffi/blitbuffer")
-        local canvas = Blitbuffer_.new(full_w, full_h, Screen.bb:getType())
+        -- KOReader's MuPDF scaler keeps the 16-bit stride when it widens RGB565 and throws, so the
+        -- reMarkable's RGB565 screen draws its thumbnails in RGB32 instead.
+        local canvas_type = Screen.bb:getType()
+        if canvas_type == Blitbuffer_.TYPE_BBRGB16 then canvas_type = Blitbuffer_.TYPE_BBRGB32 end
+        local canvas = Blitbuffer_.new(full_w, full_h, canvas_type)
         canvas:fill(INK.white)
         -- Built in a sink of its own: a thumbnail's buttons are not tappable.
         local widget = BoardUI.collectHits(build)

koreader-plugin/spec/bookorbit_board_snapshot_canvas_test.lua

@@ -0,0 +1,77 @@
+package.path = "koreader-plugin/bookorbit.koplugin/?.lua;" .. package.path
+
+local widget = {}
+function widget:extend(options)
+    local class = options or {}
+    class.__index = class
+    return setmetatable(class, { __index = self })
+end
+function widget:new(options)
+    return setmetatable(options or {}, { __index = self })
+end
+
+for _, name in ipairs({
+    "ui/geometry", "ui/widget/container/centercontainer", "ui/widget/container/framecontainer",
+    "ui/widget/container/leftcontainer", "ui/widget/container/rightcontainer",
+    "ui/widget/container/widgetcontainer", "ui/widget/horizontalgroup", "ui/widget/horizontalspan",
+    "ui/widget/overlapgroup", "ui/widget/textboxwidget", "ui/widget/textwidget",
+    "ui/widget/verticalgroup", "ui/widget/verticalspan", "ui/widget/widget",
+}) do
+    package.loaded[name] = widget
+end
+
+local Blitbuffer = { TYPE_BB8 = 1, TYPE_BBRGB16 = 5, TYPE_BBRGB24 = 6, TYPE_BBRGB32 = 7 }
+local canvases = {}
+function Blitbuffer.new(w, h, bb_type)
+    local canvas = { w = w, h = h, bb_type = bb_type, fill = function() end }
+    table.insert(canvases, canvas)
+    return canvas
+end
+package.loaded["ffi/blitbuffer"] = Blitbuffer
+
+local scaled_from = {}
+package.loaded["ui/renderimage"] = {
+    scaleBlitBuffer = function(_, bb, w, h)
+        table.insert(scaled_from, bb)
+        return { w = w, h = h, free = function() end }
+    end,
+}
+package.loaded["ui/bidi"] = {}
+package.loaded["bookorbit_board_icons"] = {}
+package.loaded["bookorbit_board_metrics"] = {
+    INK = { white = 255 },
+    px = function(value) return value end,
+}
+
+local screen_type
+local Screen = { bb = { getType = function() return screen_type end } }
+package.loaded["device"] = { screen = Screen }
+
+local BoardUI = require("bookorbit_board_ui")
+
+local function snapshotOn(bb_type, key)
+    screen_type = bb_type
+    local painted_on
+    BoardUI.snapshot(key, 1404, 1872, 300, 400, function()
+        return { paintTo = function(_, canvas) painted_on = canvas end }
+    end)
+    local canvas = canvases[#canvases]
+    assert(painted_on == canvas, key .. ": the preview is painted on the new canvas")
+    assert(scaled_from[#scaled_from] == canvas, key .. ": the painted canvas is the one scaled down")
+    return canvas.bb_type
+end
+
+-- MuPDF cannot scale RGB565: KOReader's converter keeps the 16-bit stride and the scale throws,
+-- which crashed the first-run layout picker on every reMarkable (issue 1909).
+assert(snapshotOn(Blitbuffer.TYPE_BBRGB16, "rgb565") == Blitbuffer.TYPE_BBRGB32,
+    "an RGB565 screen draws its thumbnails in RGB32")
+assert(snapshotOn(Blitbuffer.TYPE_BB8, "grey") == Blitbuffer.TYPE_BB8,
+    "an 8-bit grey screen keeps its own format")
+assert(snapshotOn(Blitbuffer.TYPE_BBRGB32, "rgb32") == Blitbuffer.TYPE_BBRGB32,
+    "a 32-bit screen keeps its own format")
+
+local count = #canvases
+BoardUI.snapshot("rgb565", 1404, 1872, 300, 400, function() error("a cached thumbnail is not rebuilt") end)
+assert(#canvases == count, "a cached thumbnail allocates no canvas")
+
+print("bookorbit_board_snapshot_canvas_test.lua: passed")