Storage: view pictures, PNG, JPEG, BMP and GIF (#45)

Enter on a picture shows it: shrunk to fit the screen, or at its own size
with Enter again and the arrows to move. Dithered to the screen's 256
colours; a colour the screen has exactly is left alone, so screenshots are
shown as they are.

The picture is decoded once, straight into the screen's buffer, and kept
there (App::retainsContent): no copy in memory. Decoding runs on the
storage task, so the keys keep working and a 12 megapixel photograph
appears as it comes instead of tripping the watchdog.

PNG, BMP and GIF are read by decoders of our own, host-tested against files
made by Pillow; the PNG one needs 32 KB where the display library's needed
44 KB in one block, which the device often doesn't have. JPEG uses the
library's TJpgDec.

Also corrects two sentences that still gave 16 KB as the editing limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/core/src/app.h

@@ -39,6 +39,13 @@ class App {
 
     virtual void draw(Canvas& canvas) = 0;
 
+    // An App whose screen is costly to draw again (a picture decoded from the card, issue #45) can
+    // keep what it drew: while this is true its part of the screen isn't cleared before draw(),
+    // which then draws only what changed. contentLost() says that it was cleared after all, or
+    // that something drawn over it has gone: everything has to be drawn again.
+    virtual bool retainsContent() const { return false; }
+    virtual void contentLost() {}
+
     void requestRedraw() { redraw_ = true; }
 
     bool consumeRedraw() {

lib/core/src/app_keys.h

@@ -292,6 +292,14 @@ inline constexpr KeyHelp kViewerOta[] = {
     {"Tab", "the file as hex"},
 };
 
+// viewer-image: A picture
+inline constexpr KeyHelp kViewerImage[] = {
+    {"Enter", "its own size, or all of it"},
+    {"; . , /", "move around it"},
+    {"i", "its size"},
+    {"Tab", "the file as hex"},
+};
+
 // notes: Notes, the list
 inline constexpr KeyHelp kNotes[] = {
     {"; .", "up, down"},