[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/cursor.c

@@ -100,7 +100,7 @@ static bool cursorTexInit(struct CursorTex * t,
   }
 
   if (!egl_shaderCompile(t->shader,
-        vertex_code, vertex_size, fragment_code, fragment_size))
+        vertex_code, vertex_size, fragment_code, fragment_size, false))
   {
     DEBUG_ERROR("Failed to compile the cursor shader");
     return false;