Lower max_color_attachments limit for GL to 4 (#6994)

Co-authored-by: Andreas Reich <r_andreas2@web.de>

Author: adrian17

CHANGELOG.md

@@ -93,6 +93,7 @@ By @brodycj in [#6924](https://github.com/gfx-rs/wgpu/pull/6924).
 - Add Flush to GL Queue::submit. By @cwfitzgerald in [#6941](https://github.com/gfx-rs/wgpu/pull/6941).
 - Fix `wgpu` not building with `--no-default-features` on when targeting `wasm32-unknown-unknown`. By @wumpf in [#6946](https://github.com/gfx-rs/wgpu/pull/6946).
 - Fix `CopyExternalImageDestInfo` not exported on `wgpu`. By @wumpf in [#6962](https://github.com/gfx-rs/wgpu/pull/6962).
+- Reduce downlevel `max_color_attachments` limit from 8 to 4 for better GLES compatibility. By @adrian17 in [#6994](https://github.com/gfx-rs/wgpu/pull/6994).
 - Fix drop order in `Surface`. By @ed-2100 in [#6997](https://github.com/gfx-rs/wgpu/pull/6997)
 - Fix a possible deadlock within `Queue::write_texture`. By @metamuffin in [#7004](https://github.com/gfx-rs/wgpu/pull/7004)
 - Fix building a BLAS with a transform buffer by adding a flag to indicate usage of the transform buffer. By @Vecvec in 

Cargo.toml

@@ -65,6 +65,7 @@ wgpu = { version = "24.0.0", path = "./wgpu", default-features = false, features
     "dx12",
     "metal",
     "static-dxc",
+    "webgl",
 ] }
 wgpu-core = { version = "24.0.0", path = "./wgpu-core" }
 wgpu-hal = { version = "24.0.0", path = "./wgpu-hal" }

wgpu-hal/src/gles/adapter.rs

@@ -825,6 +825,7 @@ impl super::Adapter {
                     private_caps,
                     workarounds,
                     features,
+                    limits: limits.clone(),
                     options: backend_options,
                     shading_language_version,
                     next_shader_id: Default::default(),

wgpu-hal/src/gles/mod.rs

@@ -274,6 +274,7 @@ struct AdapterShared {
     context: AdapterContext,
     private_caps: PrivateCapabilities,
     features: wgt::Features,
+    limits: wgt::Limits,
     workarounds: Workarounds,
     options: wgt::GlBackendOptions,
     shading_language_version: naga::back::glsl::Version,

wgpu-hal/src/gles/queue.rs

@@ -1078,8 +1078,8 @@ impl super::Queue {
                             0,
                         )
                     };
-                    for i in 0..crate::MAX_COLOR_ATTACHMENTS {
-                        let target = glow::COLOR_ATTACHMENT0 + i as u32;
+                    for i in 0..self.shared.limits.max_color_attachments {
+                        let target = glow::COLOR_ATTACHMENT0 + i;
                         unsafe {
                             gl.framebuffer_texture_2d(
                                 glow::DRAW_FRAMEBUFFER,

wgpu-types/src/lib.rs

@@ -506,7 +506,7 @@ impl Limits {
     ///     min_uniform_buffer_offset_alignment: 256,
     ///     min_storage_buffer_offset_alignment: 256,
     ///     max_inter_stage_shader_components: 60,
-    ///     max_color_attachments: 8,
+    ///     max_color_attachments: 4,
     ///     max_color_attachment_bytes_per_sample: 32,
     ///     max_compute_workgroup_storage_size: 16352, // *
     ///     max_compute_invocations_per_workgroup: 256,
@@ -526,6 +526,7 @@ impl Limits {
             max_texture_dimension_3d: 256,
             max_storage_buffers_per_shader_stage: 4,
             max_uniform_buffer_binding_size: 16 << 10, // (16 KiB)
+            max_color_attachments: 4,
             // see: https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf#page=7
             max_compute_workgroup_storage_size: 16352,
             ..Self::defaults()
@@ -563,7 +564,7 @@ impl Limits {
     ///     min_uniform_buffer_offset_alignment: 256,
     ///     min_storage_buffer_offset_alignment: 256,
     ///     max_inter_stage_shader_components: 31,
-    ///     max_color_attachments: 8,
+    ///     max_color_attachments: 4,
     ///     max_color_attachment_bytes_per_sample: 32,
     ///     max_compute_workgroup_storage_size: 0, // +
     ///     max_compute_invocations_per_workgroup: 0, // +