Move draw call to Instance

Also add some notes about error handling for adding VBO data

citro3d/examples/triangle.rs

@@ -110,7 +110,7 @@ fn main() {
                     );
                 }
 
-                target.draw_arrays(buffers::Primitive::Triangles, vbo_idx);
+                instance.draw_arrays(buffers::Primitive::Triangles, vbo_idx);
             });
         };
 

citro3d/src/buffers.rs

@@ -83,6 +83,12 @@ impl BufInfo {
         };
 
         if res < 0 {
+            // TODO: should we convert to a more specific error if this fails?
+            // It looks like the common cases are
+            // - too many buffers already added (max 12)
+            // - physical memory address in the wrong place (this can be seen by
+            //   using default allocator instead of LinearAllocator)
+            // <https://github.com/devkitPro/citro3d/blob/master/source/buffers.c#L13-L17>
             Err(crate::Error::System(res))
         } else {
             Ok(Index {

citro3d/src/lib.rs

@@ -73,6 +73,16 @@ impl Instance {
             citro3d_sys::C3D_FrameEnd(0);
         }
     }
+
+    pub fn draw_arrays(&mut self, primitive: buffers::Primitive, index: buffers::Index) {
+        unsafe {
+            citro3d_sys::C3D_DrawArrays(
+                primitive as ctru_sys::GPU_Primitive_t,
+                index.as_raw(),
+                index.size(),
+            );
+        }
+    }
 }
 
 impl Drop for Instance {

citro3d/src/render.rs

@@ -10,7 +10,7 @@ use ctru::gfx::Screen;
 use ctru::services::gspgpu::FramebufferFormat;
 use ctru_sys::{GPU_COLORBUF, GPU_DEPTHBUF};
 
-use crate::{buffers, Error, Result};
+use crate::{Error, Result};
 
 mod transfer;
 
@@ -91,16 +91,6 @@ impl<'screen> Target<'screen> {
     pub(crate) fn as_raw(&self) -> *mut C3D_RenderTarget {
         self.raw
     }
-
-    pub fn draw_arrays(&mut self, primitive: buffers::Primitive, index: buffers::Index) {
-        unsafe {
-            citro3d_sys::C3D_DrawArrays(
-                primitive as ctru_sys::GPU_Primitive_t,
-                index.as_raw(),
-                index.size(),
-            );
-        }
-    }
 }
 
 bitflags::bitflags! {