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.vert

@@ -0,0 +1,27 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision highp float;
+
+layout(binding = 0) uniform Block {
+	float z;
+	vec2 scale;
+	vec2 offset;
+	vec2 uvScale;
+	vec2 uvOffset;
+} uniforms;
+
+layout(location = 0) in vec2 pos;
+
+layout(location = 1) in vec2 uv;
+
+layout(location = 0) out vec2 vUV;
+
+void main() {
+	vec2 p = pos;
+	p *= uniforms.scale;
+	p += uniforms.offset;
+	gl_Position = vec4(p, uniforms.z, 1);
+	vUV = uv*uniforms.uvScale + uniforms.uvOffset;
+}