gg,sokol,stbi,gx,fontstash: cleanup for -Wimpure-v

vlib/sokol/sgl/sgl_structs.c.v

@@ -0,0 +1,24 @@
+module sgl
+
+// should be in a proper module
+pub enum SglError {
+	no_error
+	vertices_full
+	commands_full
+	stack_overflow
+	stack_underfloat
+}
+
+pub struct C.sgl_pipeline {
+	id u32
+}
+
+pub struct C.sgl_desc_t {
+	max_vertices       int // size for vertex buffer
+	max_commands       int // size of uniform- and command-buffers
+	pipeline_pool_size int // size of the internal pipeline pool, default is 64
+	color_format       C.sg_pixel_format
+	depth_format       C.sg_pixel_format
+	sample_count       int
+	face_winding       C.sg_face_winding // default front face winding is CCW
+}