[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/downscale_linear.frag

@@ -1,12 +1,14 @@
 #version 300 es
+#extension GL_OES_EGL_image_external_essl3 : enable
+
 precision highp float;
 
 in  vec2  fragCoord;
 out vec4  fragColor;
 
-uniform sampler2D texture;
+uniform sampler2D sampler1;
 
 void main()
 {
-  fragColor = texture2D(texture, fragCoord);
+  fragColor = texture(sampler1, fragCoord);
 }