ESP8266: shorter soft SPI fill passes to avoid soft WDT reset

Each 20000-pixel pass took about 1.65 s with no yield on a 3.5" HX8357,
which tripped the soft watchdog. 5000 pixels is about 0.4 s.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Adafruit_SPITFT.cpp

@@ -1444,8 +1444,8 @@ void Adafruit_SPITFT::writeColor(uint16_t color, uint32_t len) {
 #if defined(ESP8266)
     do {
       uint32_t pixelsThisPass = len;
-      if (pixelsThisPass > 20000)
-        pixelsThisPass = 20000;
+      if (pixelsThisPass > 5000) // About 0.4 s, well under the soft WDT
+        pixelsThisPass = 5000;
       len -= pixelsThisPass;
       optimistic_yield(10000); // Yield if 10 ms since last yield
       while (pixelsThisPass--) {