gpu/internal/shaders: generate shader variants

We're about to add Direct3D support, where shaders are written in
HLSL. Rather than write shaders twice (or more), convert them to
a GLSL variant understood by the glslcc cross-compiler and generate
the OpenGL ES 2.0 and HLSL variants. The HLSL is used by a future
change.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/blit.frag

@@ -0,0 +1,15 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision mediump float;
+
+layout(location=0) in vec2 vUV;
+
+{{.Header}}
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+	fragColor = {{.FetchColorExpr}};
+}