generalization of deferred commands and any_gl rendering where appropriate

src/rendering/twn_gl_any_rendering.c

@@ -1,4 +1,8 @@
+#include "twn_gl_any_rendering_c.h"
 #include "twn_draw_c.h"
+#include "twn_engine_context_c.h"
+
+#include <stb_ds.h>
 
 #ifdef EMSCRIPTEN
 #include <GLES2/gl2.h>
@@ -84,3 +88,35 @@ VertexBuffer get_circle_element_buffer(void) {
 
     return buffer;
 }
+
+
+/* potentially double buffered array of vertex array handles */
+/* we assume they will be refilled fully each frame */
+static size_t scratch_va_front_used, scratch_va_back_used;
+static GLuint *front_scratch_vertex_arrays, *back_scratch_vertex_arrays;
+static GLuint **current_scratch_vertex_array = &front_scratch_vertex_arrays;
+
+void restart_scratch_vertex_arrays(void) {
+    scratch_va_front_used = 0;
+    scratch_va_back_used = 0;
+
+    if (ctx.render_double_buffered) {
+        current_scratch_vertex_array = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
+                &back_scratch_vertex_arrays : &front_scratch_vertex_arrays;
+    }
+}
+
+
+GLuint get_scratch_vertex_array(void) {
+    size_t *used = current_scratch_vertex_array == &front_scratch_vertex_arrays ?
+            &scratch_va_front_used : &scratch_va_back_used;
+
+    if (arrlenu(*current_scratch_vertex_array) <= *used) {
+        GLuint handle;
+        glGenBuffers(1, &handle);
+        arrpush(*current_scratch_vertex_array, handle);
+    }
+
+    (*used)++;
+    return (*current_scratch_vertex_array)[*used - 1];
+}