Textured text calls could be invisible

Bug #6597730

Text would sometimes not appear when rendered with textured content
(BitmapShader, LinearGradientShader, etc.) This was due to a misuse
of OpenGL texture unit in FontRenderer. Textured text normally uses
two texture units:
- texture unit 0 for the font cache
- texture unit 1 for the textured content (gradient, etc.)

Recent changes to the font renderer allow it to bind new textures
while processing the text's geometry (this happens when caches get
full or when switching font size for instance.) The bindings were
done without ensuring the texture unit was the correct one
(unit 0), thus replacing the content of another texture unit
(unit 1).

This lead to text being drawn using the font cache itself as the
content texture, making the text invisible.

Change-Id: I392b4c884f09223305f6cbc6253e2ef9a98944c9