[test] Fix WebGL errors in full_es2_sdlproc.c (#27834)

`draw_gear` in `full_es2_sdlproc.c` was calling `glDrawElements` with
`GL_FLOAT` (an invalid index type in OpenGL ES 2.0 / WebGL) and passing
an integer vertex offset (`gear->strips[n].first`) as the client-side
`indices` pointer without a bound `GL_ELEMENT_ARRAY_BUFFER`.

Construct a valid `GLushort` index array for each strip and pass
`GL_UNSIGNED_SHORT` to `glDrawElements`, call `gears_reshape(300, 300)`
before `gears_draw()`, and assert `glGetError() == GL_NO_ERROR`. Also
remove the `-Wno-int-conversion` workaround from `test_fulles2_sdlproc`.

Author: sbc100

test/full_es2_sdlproc.c

@@ -497,9 +497,14 @@ draw_gear(struct gear *gear, GLfloat *transform,
    glEnableVertexAttribArray(1);
 
    /* Draw the triangle strips that comprise the gear */
-   int n;
-   for (n = 0; n < gear->nstrips; n++)
-      glDrawElements(GL_TRIANGLE_STRIP, gear->strips[n].count, GL_FLOAT, gear->strips[n].first);
+   for (int n = 0; n < gear->nstrips; n++) {
+      GLushort indices[7];
+      assert(gear->strips[n].count <= 7);
+      for (int i = 0; i < gear->strips[n].count; i++) {
+         indices[i] = gear->strips[n].first + i;
+      }
+      glDrawElements(GL_TRIANGLE_STRIP, gear->strips[n].count, GL_UNSIGNED_SHORT, indices);
+   }
 
    /* Disable the attributes */
    glDisableVertexAttribArray(1);
@@ -725,7 +730,9 @@ main(int argc, char *argv[])
 
    gears_idle();
    gears_init();
+   gears_reshape(300, 300);
    gears_draw();
+   assert(glGetError() == GL_NO_ERROR);
 
    return 0;
 }

test/test_browser.py

@@ -1668,7 +1668,7 @@ def test_glgears_animation(self, filename):
 
   @requires_graphics_hardware
   def test_fulles2_sdlproc(self):
-    self.btest_exit('full_es2_sdlproc.c', cflags=['-sGL_TESTING', '-sFULL_ES2', '-lGL', '-lSDL', '-lglut', '-sGL_ENABLE_GET_PROC_ADDRESS', '-Wno-int-conversion'])
+    self.btest_exit('full_es2_sdlproc.c', cflags=['-sGL_TESTING', '-sFULL_ES2', '-lGL', '-lSDL', '-lglut', '-sGL_ENABLE_GET_PROC_ADDRESS'])
 
   @requires_graphics_hardware
   @flaky('https://github.com/emscripten-core/emscripten/issues/25329')