rgb转yuv420p 硬件加速（opengl es,shader）

#ifdef GL_ES
precision highp float;
#endif
varying vec2 vTextureCoord;
uniform sampler2D tex; // 原始纹理
uniform float width; // 纹理宽
uniform float height; // 纹理高

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

vec2 nowTxtPos = vTextureCoord;
vec2 size = vec2(width, height);

float uvlines = 0.0625*height;   // 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));
vec2 uvPosOffset = vec2(uvlines-uvlinesI,uvlinesI/height);
vec2 uMaxPos = uvPosOffset+vec2(0,0.25);
vec2 vMaxPos = uvPosOffset+uMaxPos;

vec2 yScale = vec2(4,4);
vec2 uvScale = vec2(8,8);
// y

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