Implement get_rotation

this also stops the automatic call to `.set_rotation` in `.init`; the default
rotation is landscape.

main/paper.h

@@ -10,6 +10,11 @@
 
 #define WAVEFORM EPD_BUILTIN_WAVEFORM
 
+// this file defines lua bindings to the epdiy e-ink library.
+//
+// see https://epdiy.readthedocs.io/en/latest/api.html for epdiy documentation.
+// see https://www.lua.org/manual/5.4/manual.html for lua documentation.
+
 EpdiyHighlevelState hl;
 uint8_t* fb;
 
@@ -41,11 +46,42 @@ static int get_height (lua_State *L) {
 
 static int set_rotation (lua_State *L) {
     char *rotation = lua_tostring(L, -1);
-    luaL_argcheck(L, strcmp(rotation, "landscape") == 0 || strcmp(rotation, "portrait") == 0, -1, "expected `rotation` to be either 'landscape' or 'portrait'");
-    epd_set_rotation(strcmp(rotation, "landscape") == 0 ? EPD_ROT_LANDSCAPE : EPD_ROT_PORTRAIT);
+    luaL_argcheck(
+        L,
+        strcmp(rotation, "landscape") == 0 ||
+        strcmp(rotation, "portrait") == 0 ||
+        strcmp(rotation, "inverted_landscape") == 0 ||
+        strcmp(rotation, "inverted_portrait") == 0,
+        -1,
+        "expected `rotation` to be either 'landscape', 'portrait', 'inverted_landscape' or 'inverted_portrait'"
+    );
+    epd_set_rotation(
+        strcmp(rotation, "landscape") == 0
+        ? EPD_ROT_LANDSCAPE
+        : strcmp(rotation, "portrait") == 0
+        ? EPD_ROT_PORTRAIT
+        : strcmp(rotation, "inverted_landscape") == 0
+        ? EPD_ROT_INVERTED_LANDSCAPE
+        : EPD_ROT_INVERTED_PORTRAIT
+    );
     return 0;
 }
 
+static int get_rotation (lua_State *L) {
+    int rotation = epd_get_rotation();
+    lua_pushstring(
+        L,
+        rotation == EPD_ROT_LANDSCAPE
+        ? "landscape"
+        : rotation == EPD_ROT_PORTRAIT
+        ? "portrait"
+        : rotation == EPD_ROT_INVERTED_LANDSCAPE
+        ? "inverted_landscape"
+        : "inverted_portrait"
+    );
+    return 1;
+}
+
 static int draw_pixel (lua_State *L) {
     int x, y, color;
     int isnum;
@@ -260,6 +296,7 @@ static const struct luaL_Reg paper[] = {
   {"get_height", get_height},
 
   {"set_rotation", set_rotation},
+  {"get_rotation", get_rotation},
 
   {"draw_pixel", draw_pixel},
   {"draw_line", draw_line},