[client] egl: switch to using GL_TEXTURE_EXTERNAL_OES for DMABUF

According to Erik @ NVidia the open source NVidia driver will not
create a EGLImage from a DMABUF if the target is not
GL_TEXTURE_EXTERNAL_OES. This change set converts the dmabuf texture
from GL_TEXTURE_2D to GL_TEXTURE_EXTERNAL_OES and at runtime performs a
global search & replace on fragment shaders as needed to remain
compatible, replacing `sampler2D` with `samplerExternalOES`.

Ref: https://github.com/NVIDIA/open-gpu-kernel-modules/discussions/243#discussioncomment-3283415

client/renderers/EGL/shader/desktop_rgb.frag

@@ -1,4 +1,6 @@
 #version 300 es
+#extension GL_OES_EGL_image_external_essl3 : enable
+
 precision highp float;
 
 #define EGL_SCALE_AUTO    0
@@ -23,13 +25,17 @@ void main()
   switch (scaleAlgo)
   {
     case EGL_SCALE_NEAREST:
+    {
       vec2 ts = vec2(textureSize(sampler1, 0));
       color   = texelFetch(sampler1, ivec2(uv * ts), 0);
       break;
+    }
 
     case EGL_SCALE_LINEAR:
+    {
       color = texture(sampler1, uv);
       break;
+    }
   }
 
   if (cbMode > 0)