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/intersect.frag

@@ -0,0 +1,18 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision mediump float;
+
+// Use high precision to be pixel accurate for
+// large cover atlases.
+layout(location = 0) in highp vec2 vUV;
+
+layout(binding = 0) uniform sampler2D cover;
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+  float cover = abs(texture(cover, vUV).r);
+  fragColor.r = cover;
+}