renderer/opengl: Extract shaders from source (#9600)

---------

Co-authored-by: Mihai Fufezan <mihai@fufexan.net>

src/render/shaders/glsl/rgba.frag

@@ -0,0 +1,36 @@
+#extension GL_ARB_shading_language_include : enable
+precision highp float;
+varying vec2 v_texcoord; // is in 0-1
+uniform sampler2D tex;
+uniform float alpha;
+
+#include "rounding.glsl"
+
+uniform int discardOpaque;
+uniform int discardAlpha;
+uniform float discardAlphaValue;
+
+uniform int applyTint;
+uniform vec3 tint;
+
+void main() {
+
+    vec4 pixColor = texture2D(tex, v_texcoord);
+
+    if (discardOpaque == 1 && pixColor[3] * alpha == 1.0)
+	    discard;
+
+    if (discardAlpha == 1 && pixColor[3] <= discardAlphaValue)
+        discard;
+
+    if (applyTint == 1) {
+	    pixColor[0] = pixColor[0] * tint[0];
+	    pixColor[1] = pixColor[1] * tint[1];
+	    pixColor[2] = pixColor[2] * tint[2];
+    }
+
+    if (radius > 0.0)
+    	pixColor = rounding(pixColor);
+
+    gl_FragColor = pixColor * alpha;
+}