Improved font caching

I made a patch that improves the performance of font caching mechanism.
This is based on a funny behaviour of FontConfig: it was handling
FcCharSet in a somewhat unexpected way.

So, we are currently adding "a character" to a new FcCharSet, and then
add it to a FcPattern. However, if we toss the FcPattern to FontConfig,
it loads the entire language(charset) that contains the character we
gave. That is, we don't always have to load a new font for each unknown
character. Instead, we can reused cached fonts, and this significantly
reduces the number of calls to extremely slow FontConfig matching
functions.

One more thing. I found that, in libXft, there's a function called
XftCharExists. XftCharIndex internally calls this function, and
does more stuffs if the character does exist. Since the returned index
is never used in st, we should call XftCharExists instead of
XftCharIndex. Please note that I already made this change in the patch.
Add RGB color definition

This patch uses the bit 24 in the color descriptor as an indicator
of RGB color, so we can take the values and generating the XftColour
directly in xdraws.
Reload colors in reset

Colors definition can be changed using a OSC sequence, so
we have to reload them if we want be sure all the colors
are the correct.

Could be desirable free the colors allocated due to rgb
colors and inverse colors (XftColorAllocValues in xdraws),
but it is impossible due we use the same structure for all
of them.
Merge branch 'master' into unstable-argb
Use character size scaling factors

The bounding boxes for characters can be scaled using "cwscale" and "chscale"
to scale the width and height respectively.
Fix blink mode check

ATTR_BLINK is an attribute for a Glyph and will not be set in term.mode.
Fixed memory leak in xsettitle().
Fix signess of tdefcolor

tdefcolor() returns -1 on error, while its return type is
unsigned long. At the same time, line 1724 and 1731 are checking the
positivity of its unsigned return value.
Merge branch 'master' into unstable-argb

Conflicts:
	st.c