add file picking for osx and ios and implement equirectangular import

engine/texture.h

@@ -13,6 +13,7 @@ public:
     bool create(const ui::Image& img);
     void assign(GLuint tex, int w = -1, int h = -1, GLuint internal_format = GL_RGBA8, GLuint format = GL_RGBA);
     bool load(std::string filename);
+    bool load_file(std::string filename);
     void destroy() { if (m_tex) LOG("TEX destroy %d", m_tex); glDeleteTextures(1, &m_tex); }
     void bind() const { glBindTexture(GL_TEXTURE_2D, m_tex); }
     void unbind() const { glBindTexture(GL_TEXTURE_2D, 0); }