I'm trying to render text with OpenGL and an "inline"-font taken from the stb_fonts

The relevant code for initializing the font & rendering:

LabelFactory::LabelFactory() {

   static unsigned char fontpixels [STB_SOMEFONT_BITMAP_HEIGHT][STB_SOMEFONT_BITMAP_WIDTH];
   STB_SOMEFONT_CREATE(fontdata, fontpixels, STB_SOMEFONT_BITMAP_HEIGHT);

   glGenTextures(1, &texture);
   glBindTexture(GL_TEXTURE_2D, texture);
   glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

   glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, STB_SOMEFONT_BITMAP_WIDTH, STB_SOMEFONT_BITMAP_HEIGHT, 0, GL_ALPHA, GL_UNSIGNED_BYTE, fontdata);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
}
void LabelFactory::renderLabel(Label * label) {
   int x = label->x;
   int y = label->y;
   const char * str = label->text;

   glBindTexture(GL_TEXTURE_2D, texture);

   glEnable(GL_BLEND);
   glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
   glEnable(GL_ALPHA_TEST);
   glColor4f(1.0f, 1.0f, 1.0f, 1.0f);

   glEnable(GL_TEXTURE_2D);

   glBegin(GL_QUADS);
   while (*str) {
     int char_codepoint = *str++;
     stb_fontchar *cd = &fontdata[char_codepoint - STB_FONT_arial_14_usascii_FIRST_CHAR];
     glTexCoord2f(cd->s0, cd->t0); glVertex2i(x + cd->x0, y + cd->y0);
     glTexCoord2f(cd->s1, cd->t0); glVertex2i(x + cd->x1, y + cd->y0);
     glTexCoord2f(cd->s1, cd->t1); glVertex2i(x + cd->x1, y + cd->y1);
     glTexCoord2f(cd->s0, cd->t1); glVertex2i(x + cd->x0, y + cd->y1);
     x += cd->advance_int;
   }   

   glEnd();
}  

However this results in weird glitches

I guess I'm doing something wrong with the alpha blending, however I was unable to improve it by changing the parameters. The size and length of the outline of the text that should be shown seems about right (it should read "Test Test Test").