Don't invalidate all the glyphs when flushing large textures

FontRenderer::flushLargeCaches identifies the large textures used to
cache glyphs and visits all the known fonts to mark their glyphs
invalid if they belong to one of these large textures.

Unfortunately, Font::invalidateTextureCache had a logic error
which would make it mark *all glyphs* as invalid, not matter
what texture they belong to. This means that any large cache
flush would cause all glyphs to be invalidate, thus forcing
the rendering system to recreate them on the next draw.

Font::invalidateTextureCache is supposed to behave this way:
- If the specified cacheTexture is NULL (default value), mark
all glyphs as invalid (see FontRenderer::flushAllAndInvalidate())
- If cacheTexture is *not* NULL, invalidate only the glyphs for
which glyph.cacheTexture == cacheTexture.

The previous condition read:

if (cacheTexture || glyph.cacheTexture == cacheTexture)

This test *always* passes.

Change-Id: I418886cb594c81c6178d0f9e9953d975e991cf22