spirv: Support initializers on uniforms (#1588)

If a uniform has an initializer it will now be given as the optional
initializer operand to the OpVariable instruction.

Fixes: https://github.com/KhronosGroup/glslang/issues/1259

Signed-off-by: Neil Roberts <nroberts@igalia.com> (the code)
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com> (the tests)
Signed-off-by: Arcady Goldmints-Orlov <agoldmints@igalia.com>

Co-authored-by: Neil Roberts <nroberts@igalia.com>

Test/spv.uniformInitializer.frag

@@ -0,0 +1,10 @@
+#version 450
+
+layout (location = 0) out vec4 color;
+
+layout (location = 0) uniform vec4 in_color = vec4(0.0, 1.0, 0.0, 1.0);
+
+void main()
+{
+	color = in_color;
+}