use main buffer and copy data to fbo texture (opengl) (#5294)

NEEDS REVIEW

continuation of #5037
resolves #4729 

renders all shaders to the default buffer and then copies it to the
designated custom shader texture.

this is a draft pr because:
- it introduces a new shader "pipeline" which doesnt fit in with how the
system was designed to work (which is only rendering to the fbo)
- im not sure if this is the best way to achieve shaders being able to
sample their output while also drawing to the screen. the cusom fbo
(previous implementation) was useful in that it modularized the custom
shader stage in rendering

---------

Co-authored-by: Mitchell Hashimoto <m@mitchellh.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/pkg/opengl/Texture.zig b/pkg/opengl/Texture.zig
index 4cd1cf9f9..a9fa5d4fe 100644
--- a/pkg/opengl/Texture.zig
+++ b/pkg/opengl/Texture.zig
@@ -162,4 +162,26 @@ pub const Binding = struct {
             data,
         );
     }
+
+    pub fn copySubImage2D(
+        b: Binding,
+        level: c.GLint,
+        xoffset: c.GLint,
+        yoffset: c.GLint,
+        x: c.GLint,
+        y: c.GLint,
+        width: c.GLsizei,
+        height: c.GLsizei,
+    ) !void {
+        glad.context.CopyTexSubImage2D.?(
+            @intFromEnum(b.target),
+            level,
+            xoffset,
+            yoffset,
+            x,
+            y,
+            width,
+            height
+        );
+    }
 };