rgb转yuv420sp_nv12 硬件加速（opengl es,shader）

static const char frag_RGB_to_YUV420SP_NV12[] =
"#ifdef GL_ES\n"
"precision highp float;\n"
"#endif\n"
"varying vec2 v_texcoord;\n"
"uniform sampler2D tex;\n"
"uniform float width;\n"
"uniform float height;\n"

"void main(void) {\n"
" vec3 offset = vec3(0.0625, 0.5, 0.5);\n"
" vec3 ycoeff = vec3(0.256816, 0.504154, 0.0979137);\n"
" vec3 ucoeff = vec3(-0.148246, -0.29102, 0.439266);\n"
" vec3 vcoeff = vec3(0.439271, -0.367833, -0.071438);\n"

" vec2 nowTxtPos = v_texcoord;\n"
" vec2 size = vec2(width, height);\n"

" float uvlines = 0.125*height;\n"   // 0.625:w*h/4(uv数据所占内存空间) / w*h*4(rgba总空间) = 0.625(u或v数据在当前fbo中所占比例);uvlines:uv数据需要多少行
" float uvlinesI = float(int(uvlines));\n"
" vec2 uvPosOffset = vec2(uvlines-uvlinesI,uvlinesI/height);\n"
" vec2 uvMaxPos = uvPosOffset+vec2(0,0.25);\n"

" vec2 yScale = vec2(4,4);\n"
" vec2 uvScale = vec2(4,4);\n"
// y

" if(nowTxtPos.y<0.25){\n"
// y base postion