GPUBatch: Add draw parameter getter

This is used to populate indirect draw commands in the draw manager.

source/blender/gpu/GPU_batch.h

@@ -164,6 +164,13 @@ void GPU_batch_program_set_builtin_with_config(GPUBatch *batch,
 #define GPU_batch_texture_bind(batch, name, tex) \
   GPU_texture_bind(tex, GPU_shader_get_texture_binding((batch)->shader, name));
 
+/**
+ * Return indirect draw call parameters for this batch.
+ * NOTE: r_base_index is set to -1 if not using an index buffer.
+ */
+void GPU_batch_draw_parameter_get(
+    GPUBatch *batch, int *r_v_count, int *r_v_first, int *r_base_index, int *r_i_count);
+
 void GPU_batch_draw(GPUBatch *batch);
 void GPU_batch_draw_range(GPUBatch *batch, int v_first, int v_count);
 /**

source/blender/gpu/intern/gpu_batch.cc

@@ -220,6 +220,30 @@ void GPU_batch_set_shader(GPUBatch *batch, GPUShader *shader)
 /** \name Drawing / Drawcall functions
  * \{ */
 
+void GPU_batch_draw_parameter_get(
+    GPUBatch *gpu_batch, int *r_v_count, int *r_v_first, int *r_base_index, int *r_i_count)
+{
+  Batch *batch = static_cast<Batch *>(gpu_batch);
+
+  if (batch->elem) {
+    *r_v_count = batch->elem_()->index_len_get();
+    *r_v_first = batch->elem_()->index_start_get();
+    *r_base_index = batch->elem_()->index_base_get();
+  }
+  else {
+    *r_v_count = batch->verts_(0)->vertex_len;
+    *r_v_first = 0;
+    *r_base_index = -1;
+  }
+
+  int i_count = (batch->inst[0]) ? batch->inst_(0)->vertex_len : 1;
+  /* Meh. This is to be able to use different numbers of verts in instance VBO's. */
+  if (batch->inst[1] != nullptr) {
+    i_count = min_ii(i_count, batch->inst_(1)->vertex_len);
+  }
+  *r_i_count = i_count;
+}
+
 void GPU_batch_draw(GPUBatch *batch)
 {
   GPU_shader_bind(batch->shader);

source/blender/gpu/intern/gpu_index_buffer_private.hh

@@ -70,6 +70,14 @@ class IndexBuf {
      * They can lead to graphical glitches on some systems. (See T96892) */
     return is_empty_ ? 0 : index_len_;
   }
+  uint32_t index_start_get() const
+  {
+    return index_start_;
+  }
+  uint32_t index_base_get() const
+  {
+    return index_base_;
+  }
   /* Return size in byte of the drawable data buffer range. Actual buffer size might be bigger. */
   size_t size_get() const
   {