examples: add .obj viewer as a sokol example (#9456)

examples/sokol/06_obj_viewer/obj/struct.v

@@ -0,0 +1,98 @@
+/**********************************************************************
+*
+* .obj loader
+*
+* Copyright (c) 2021 Dario Deledda. All rights reserved.
+* Use of this source code is governed by an MIT license
+* that can be found in the LICENSE file.
+*
+* TODO:
+**********************************************************************/
+module obj
+import gg.m4
+
+// part struct mantain the fae indexes list
+pub struct Part {
+pub mut:
+	faces     [][][3]int // v n t index order, if -1 not available
+	name      string
+	material  string
+}
+
+// materias struct, all Ks and Ns are stored as maps of string
+pub struct Material {
+pub mut:
+	name     string
+	ks       map[string]m4.Vec4
+	ns       map[string]f32
+	maps     map[string]string
+}
+
+// render data used for the rendering
+pub struct Render_data{
+pub mut:
+	pipeline C.sg_pipeline
+	bind     C.sg_bindings
+	n_vert   u32
+	material string
+}
+
+// base object parts struct
+pub struct ObjPart {
+pub mut:
+	v    []m4.Vec4  // position
+	vn   []m4.Vec4  // normals
+	vp   []m4.Vec4  // vertex params
+	vt   []m4.Vec4  // textures
+	
+	name    string
+	part    []Part                // parts of the ObjPart
+	mat     []Material            // list of the materials of the ObjPart
+	mat_map map[string]int        // maping material name to its material index
+	texture map[string]C.sg_image // GPU loaded texture map
+	material_file string          // .mtl file name for the .obj
+		
+	rend_data []Render_data       // render data used for the rendering
+	
+	t_m  m4.Mat4 = m4.unit_m4() // transform matrix for this ObjPart
+	//child []ObjPart           // childs
+	
+	// stats
+	min    m4.Vec4   // min 3d position in the ObjPart
+	max    m4.Vec4   // max 3d position in the ObjPart
+	radius f32       // bounding circle radius of the ObjPart
+}
+
+// used in to pass the matrices to the shader
+pub struct Mats {
+pub mut:
+	mv   m4.Mat4
+	mvp  m4.Mat4
+	nm   m4.Mat4
+}
+
+// data passed to the vertex shader
+pub struct Tmp_vs_param {
+pub mut:
+	mv m4.Mat4
+	mvp m4.Mat4
+	nm  m4.Mat4
+}
+
+// data passed to the pixel shader
+pub struct Tmp_fs_param {
+pub mut:
+	ligth m4.Vec4
+	ka    m4.Vec4 = m4.Vec4{e:[f32(0.1), 0.0, 0.0, 1.0]!}
+	kd    m4.Vec4 = m4.Vec4{e:[f32(0.5), 0.5, 0.5, 1.0]!}
+	ks    m4.Vec4 = m4.Vec4{e:[f32(1.0), 1.0, 1.0, 1.0]!}
+}
+
+// shader data for the rendering
+pub struct Shader_data {
+pub mut:
+	vs_data &Tmp_vs_param
+	vs_len  int
+	fs_data &Tmp_fs_param
+	fs_len  int
+}