Preserve wide custom-font ranges on big-endian targets

GFXfont stores first and last as uint16_t, but the custom-font paths read them as bytes. Use word-width reads while retaining the existing 8-bit character API, so the stored range is interpreted correctly without changing the font format or adding UTF-8 behavior.

Constraint: Keep the public 8-bit character API and limit the patch to issue #490\nRejected: UTF-8/API refactor | broader scope is outside the reported portability bug\nConfidence: high\nScope-risk: narrow\nReversibility: clean\nDirective: Keep GFXfont first/last accesses word-width when reading the stored range\nTested: MinGW GCC 8 -Wall -Wextra -Werror with an endian-aware host fixture; WSL GCC AddressSanitizer and UBSan; baseline fixture fails with 0x0 bounds while the fix passes with 5x7\nNot-tested: Physical Arduino board hardware

Adafruit_GFX.cpp

@@ -1435,7 +1435,7 @@ void Adafruit_GFX::drawChar(int16_t x, int16_t y, unsigned char c,
     // newlines, returns, non-printable characters, etc.  Calling
     // drawChar() directly with 'bad' characters of font may cause mayhem!
 
-    c -= (uint8_t)pgm_read_byte(&gfxFont->first);
+    c -= (uint8_t)pgm_read_word(&gfxFont->first);
     GFXglyph *glyph = pgm_read_glyph_ptr(gfxFont, c);
     uint8_t *bitmap = pgm_read_bitmap_ptr(gfxFont);
 
@@ -1519,8 +1519,8 @@ size_t Adafruit_GFX::write(uint8_t c) {
       cursor_y +=
           (int16_t)textsize_y * (uint8_t)pgm_read_byte(&gfxFont->yAdvance);
     } else if (c != '\r') {
-      uint8_t first = pgm_read_byte(&gfxFont->first);
-      if ((c >= first) && (c <= (uint8_t)pgm_read_byte(&gfxFont->last))) {
+      uint16_t first = pgm_read_word(&gfxFont->first);
+      if ((c >= first) && (c <= pgm_read_word(&gfxFont->last))) {
         GFXglyph *glyph = pgm_read_glyph_ptr(gfxFont, c - first);
         uint8_t w = pgm_read_byte(&glyph->width),
                 h = pgm_read_byte(&glyph->height);
@@ -1635,8 +1635,8 @@ void Adafruit_GFX::charBounds(unsigned char c, int16_t *x, int16_t *y,
       *x = 0;        // Reset x to zero, advance y by one line
       *y += textsize_y * (uint8_t)pgm_read_byte(&gfxFont->yAdvance);
     } else if (c != '\r') { // Not a carriage return; is normal char
-      uint8_t first = pgm_read_byte(&gfxFont->first),
-              last = pgm_read_byte(&gfxFont->last);
+      uint16_t first = pgm_read_word(&gfxFont->first),
+               last = pgm_read_word(&gfxFont->last);
       if ((c >= first) && (c <= last)) { // Char present in this font?
         GFXglyph *glyph = pgm_read_glyph_ptr(gfxFont, c - first);
         uint8_t gw = pgm_read_byte(&glyph->width),