renderer: disable multi-buffering for OpenGL

Frames are sequential for OpenGL since the completion handler always
calls `glFinish`, so the extra buffers do nothing but waste memory.

src/renderer/OpenGL.zig

@@ -29,6 +29,10 @@ pub const imagepkg = @import("opengl/image.zig");
 
 pub const custom_shader_target: shadertoy.Target = .glsl;
 
+/// Because OpenGL's frame completion is always
+/// sync, we have no need for multi-buffering.
+pub const swap_chain_count = 1;
+
 const log = std.log.scoped(.opengl);
 
 /// We require at least OpenGL 4.3

src/renderer/generic.zig

@@ -204,8 +204,15 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
             // If this is one then we don't do any double+ buffering at all.
             // This is comptime because there isn't a good reason to change
             // this at runtime and there is a lot of complexity to support it.
-            // For comptime, this is useful for debugging.
-            const buf_count = 3;
+            const buf_count = count: {
+                if (@hasDecl(GraphicsAPI, "swap_chain_count")) {
+                    break :count GraphicsAPI.swap_chain_count;
+                }
+
+                // Default to triple buffering if
+                // graphics API has no preference.
+                break :count 3;
+            };
 
             /// `buf_count` structs that can hold the
             /// data needed by the GPU to draw a frame.