Use premultiplied alpha for renderer clearColor

[lixiaoyan]

Fixes #3324

---

I haven't tested it on Linux yet, but I believe it has the similar problem and could be fixed by this PR as well.

[SimonYde]

I cannot get this to build on my linux machine using flake.nix:

>    └─ zig build-exe ghostty Debug native-native 1 errors
┃        > src/renderer/OpenGL.zig:2347:63: error: expected type 'f32', found 'f64'
┃        >         @as(f32, @floatFromInt(self.draw_background.r)) / 255 * self.config.background_opacit…
┃        >         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
┃        > pkg/opengl/draw.zig:5:22: note: parameter type declared here
┃        > pub fn clearColor(r: f32, g: f32, b: f32, a: f32) void {
┃        >                      ^~~
┃        > referenced by:
┃        >     drawFrame: src/renderer/OpenGL.zig:2289:13
┃        >     render: src/apprt/gtk/Surface.zig:669:35
┃        >     remaining reference traces hidden; use '-freference-trace' to see all reference traces
┃        > error: the following command failed with 1 compilation errors:
┃        > /nix/store/v9xybfsc2iyfd602s192pymyrrncni0n-zig-0.13.0/bin/zig build-exe /build/source/.zig-c…
┃        > Build Summary: 94/97 steps succeeded; 1 failed (disable with --summary none)
┃        > install transitive failure
┃        > └─ install ghostty transitive failure
┃        >    └─ zig build-exe ghostty Debug native-native 1 errors

Appears to be a type mismatch issue.

[gerblesh]

I cannot get this to build on my linux machine using flake.nix:

>    └─ zig build-exe ghostty Debug native-native 1 errors
┃        > src/renderer/OpenGL.zig:2347:63: error: expected type 'f32', found 'f64'
┃        >         @as(f32, @floatFromInt(self.draw_background.r)) / 255 * self.config.background_opacit…
┃        >         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
┃        > pkg/opengl/draw.zig:5:22: note: parameter type declared here
┃        > pub fn clearColor(r: f32, g: f32, b: f32, a: f32) void {
┃        >                      ^~~
┃        > referenced by:
┃        >     drawFrame: src/renderer/OpenGL.zig:2289:13
┃        >     render: src/apprt/gtk/Surface.zig:669:35
┃        >     remaining reference traces hidden; use '-freference-trace' to see all reference traces
┃        > error: the following command failed with 1 compilation errors:
┃        > /nix/store/v9xybfsc2iyfd602s192pymyrrncni0n-zig-0.13.0/bin/zig build-exe /build/source/.zig-c…
┃        > Build Summary: 94/97 steps succeeded; 1 failed (disable with --summary none)
┃        > install transitive failure
┃        > └─ install ghostty transitive failure
┃        >    └─ zig build-exe ghostty Debug native-native 1 errors

Appears to be a type mismatch issue.

Should be fixed with the above suggestion

[SimonYde]

works just fine on linux now (as in it appears to fix #3324) 👍

[yspreen]

works wonders on macOS. does fix #3324