Fonts should have const data.

PiperOrigin-RevId: 836624808
Change-Id: I984c94cc34ef22b875ddda87c59893aa7f38a8a3

src/experimental/filament/render_context_filament.cc

@@ -137,8 +137,8 @@ void mjr_readPixels(unsigned char* rgb, float* depth, mjrRect viewport,
   g_filament_context->ReadPixels(viewport, rgb, depth);
 }
 
-void mjr_uploadFont(unsigned char* pixels, int width, int height, int bpp,
-                          int id, const mjrContext* con) {
+void mjr_uploadFont(const unsigned char* pixels, int width, int height, int bpp,
+                    int id, const mjrContext* con) {
   CheckFilamentContext();
   if (bpp != 4) {
     mju_error("Only 4bpp fonts are supported, got %d", bpp);

src/experimental/filament/render_context_filament.h

@@ -70,7 +70,7 @@ MJAPI void mjr_uploadMesh(const mjModel* m, const mjrContext* con, int meshid);
 
 MJAPI void mjr_uploadTexture(const mjModel* m, const mjrContext* con, int texid);
 
-void mjr_uploadFont(unsigned char* pixels, int width, int height, int bpp,
+void mjr_uploadFont(const unsigned char* pixels, int width, int height, int bpp,
                     int id, const mjrContext* con);
 
 MJAPI void mjr_setBuffer(int framebuffer, mjrContext* con);