[Grayscale::getShaderCode] fixed near/far clipping

use far_value as distance from camera. (not from near_value)
added glInternalFormat arg in tex.allocate

glInternalFormatの指定が必要なようでした。
Merge pull request #4 from nariakiiwatani/bugfix-near_far_clipping

[Grayscale::getShaderCode] fixed near/far clipping
Merge pull request #5 from nariakiiwatani/bugfix-depth_tex_allocating

added glInternalFormat arg in tex.allocate
Change: return bool if device can not initialized