Make 3 the default number of channels in mjsTexture.

PiperOrigin-RevId: 679521155
Change-Id: Ie6c962e75983c8bf8323170338b3d480be3a1117

src/user/user_init.c

@@ -274,6 +274,7 @@ void mjs_defaultTexture(mjsTexture* texture) {
   texture->rgb2[0] = texture->rgb2[1] = texture->rgb2[2] = 0.5;
   texture->random = 0.01;
   texture->gridsize[0] = texture->gridsize[1] = 1;
+  texture->nchannel = 3;
   char defaultlayout[sizeof(texture->gridlayout)] = "............";
   strncpy(texture->gridlayout, defaultlayout, sizeof(texture->gridlayout));
 }

src/xml/xml_native_reader.cc

@@ -3236,9 +3236,7 @@ void mjXReader::Asset(XMLElement* section, const mjVFS* vfs) {
       }
       ReadAttrInt(elem, "width", &texture->width);
       ReadAttrInt(elem, "height", &texture->height);
-      if (!ReadAttrInt(elem, "nchannel", &texture->nchannel)) {
-        texture->nchannel = 3;
-      }
+      ReadAttrInt(elem, "nchannel", &texture->nchannel);
       ReadAttr(elem, "rgb1", 3, texture->rgb1, text);
       ReadAttr(elem, "rgb2", 3, texture->rgb2, text);
       ReadAttr(elem, "markrgb", 3, texture->markrgb, text);