GL_ARB_shader_texture is supported on dx9 ATI cards(and probably dx10
ones too). For Nvidia cards I included a fallback to normal texld.
GL_EXT_gpu_shader4 supports similar texture*Grad GLSL functions, just
with an EXT prefix instead of ARB. For dx9 NV cards we'd have to use
GL_NV_fragment_program2, which supports a texldd equivalent on those
cards.