Add outline pass to cubes

shaders/src/cube.glsl

@@ -0,0 +1,41 @@
+@header package shaders
+@header import sg "../../sokol/gfx"
+
+@ctype mat4 Mat4
+
+@vs vs_cube
+in vec3 pos;
+in vec2 uv;
+
+layout(binding = 0) uniform VsParamsCube {
+    mat4 mvp;
+    vec4 col;
+};
+
+out vec4 color;
+out vec2 tex_coord;
+
+void main() {
+    gl_Position = mvp * vec4(pos, 1);
+    color = col;
+    tex_coord = uv;
+}
+
+@end
+
+@fs fs_cube
+in vec4 color;
+in vec2 tex_coord;
+
+layout(binding=0) uniform texture2D tex;
+layout(binding=0) uniform sampler smp;
+
+out vec4 frag_color;
+
+void main() {
+    frag_color = texture(sampler2D(tex, smp), tex_coord) * color;
+}
+
+@end
+
+@program cube vs_cube fs_cube