In OpenGL ES 3, glTexImage2D cannot handle GL_BGRA format. SDL_ttf
returns pixels in BGRA format (because it's the format used in
FreeType2), so it needs to be converted to RGBA before passing to
gfx_init_texture_with_pixels.
Enough to get text on the screen, but it isn't pretty. No antialiasing,
kerning, vertical placement isn't correct, probably the size isn't even
quite correct, etc.