glgsg: Force nearest filtering on isampler/usampler

Without this, Intel drivers will sample (0, 0, 0, 1)

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -12409,10 +12409,20 @@ specify_texture(CLP(TextureContext) *gtc, const SamplerState &sampler) {
     }
   }
 
+  if (Texture::is_integer(tex->get_format())) {
+    // Integer format textures can't have filtering enabled, and in fact, some
+    // drivers (looking at you, Intel) will always sample (0, 0, 0, 1) if we
+    // don't set this correctly!
+    glTexParameteri(target, GL_TEXTURE_MIN_FILTER,
+                    uses_mipmaps ? GL_NEAREST_MIPMAP_NEAREST
+                                 : GL_NEAREST);
+    glTexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+  } else {
     glTexParameteri(target, GL_TEXTURE_MIN_FILTER,
                     get_texture_filter_type(minfilter, !uses_mipmaps));
     glTexParameteri(target, GL_TEXTURE_MAG_FILTER,
                     get_texture_filter_type(magfilter, true));
+  }
 
   // Set anisotropic filtering.
   if (_supports_anisotropy) {

panda/src/glstuff/glShaderContext_src.cxx

@@ -2849,6 +2849,11 @@ update_shader_texture_bindings(ShaderContext *prev) {
       }
       continue;
     }
+    else if (Texture::is_integer(tex->get_format())) {
+      // Required to satisfy Intel drivers, which will otherwise sample zero.
+      sampler.set_minfilter(sampler.uses_mipmaps() ? SamplerState::FT_nearest_mipmap_nearest : SamplerState::FT_nearest);
+      sampler.set_magfilter(SamplerState::FT_nearest);
+    }
 
     if (tex->get_texture_type() != spec._desired_type) {
       switch (spec._part) {

panda/src/gobj/texture.cxx

@@ -2628,6 +2628,25 @@ is_srgb(Format format) {
   }
 }
 
+/**
+ * Returns true if the indicated format is an integer format, false otherwise.
+ */
+bool Texture::
+is_integer(Format format) {
+  switch (format) {
+  case F_r32i:
+  case F_r8i:
+  case F_rg8i:
+  case F_rgb8i:
+  case F_rgba8i:
+  case F_r16i:
+    return true;
+
+  default:
+    return false;
+  }
+}
+
 /**
  * Computes the proper size of the texture, based on the original size, the
  * filename, and the resizing whims of the config file.

panda/src/gobj/texture.h

@@ -625,6 +625,7 @@ public:
   static bool has_alpha(Format format);
   static bool has_binary_alpha(Format format);
   static bool is_srgb(Format format);
+  static bool is_integer(Format format);
 
   static bool adjust_size(int &x_size, int &y_size, const std::string &name,
                           bool for_padding, AutoTextureScale auto_texture_scale = ATS_unspecified);