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.
Fix some bugs in mouse tracking logic

* Button number in X10 mode:

  I believe the button - 1 came from "C b is button - 1" from [0].
  However, above this section, it states

    "Normally, parameters (such as pointer poisition and button number)
     for all mouse tracking escape sequences generated by xterm encode
     numeric parameters in a single character as value+32. For example, !
     specifies the value 1."

  Also, from the description of SGR,

    "The encoded button value in this case does not add 32 since that
     was useful only in the X10 scheme for ensuring that the byte
     containing the button value is a printable code."

  This suggests that we should still add 32 to the button value when in
  MODE_MOUSEX10.

* No button release reporting in X10 mode:

    "X10 compatibility mode sends an escape sequence only on button press,
     encoding the location and the mouse button pressed."

* Fix MODE_MOUSEMOTION:

  Currently, motion reporting is skipped when oldbutton == 3
  (corresponding to no button being pressed). However, oldbutton is
  only set on a button press, which will never be 3.

[0]: http://invisible-island.net/xterm/ctlseqs/ctlseqs.html
Fix blink mode check

ATTR_BLINK is an attribute for a Glyph and will not be set in term.mode.