Performance improvement of font generation

Because texture is frequently generated, performance is poor.
The By setting the charcodes in advance, and improved the performance
greatly.

test textbox.lua(demo)

* Before
TEXTURE: '+'       2748 =   0.00MB < /Users/mkinoshita/moai-work/moaigui/resources/gui/vert scroll center.png
TEXTURE: '+'      32768 =   0.03MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'        340 =   0.03MB < /Users/mkinoshita/moai-work/moaigui/resources/gui/vert scroller.png
TEXTURE: '+'      32768 =   0.07MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.10MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.13MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.16MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.19MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.22MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.25MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.28MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.32MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'      32768 =   0.35MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF

* After
TEXTURE: '+'       2748 =   0.00MB < /Users/mkinoshita/moai-work/moaigui/resources/gui/vert scroll center.png
TEXTURE: '+'      32768 =   0.03MB < /Users/mkinoshita/moai-work/moaigui/resources/fonts/arial-rounded.TTF
TEXTURE: '+'        340 =   0.03MB < /Users/mkinoshita/moai-work/moaigui/resources/gui/vert scroller.png